There's a Death Cab for Cutie tour documentary, by the name of Drive Well, Sleep Carefully, that came out around a decade ago. In retrospect, we know now that the period it captures - following the band as they support their critical and commercial breakthrough, Transatlanticism - was the most important of their career, as well as roughly being the point at which frontman Ben Gibbard was at the peak of his creative powers; he'd written that record and The Postal Service's Give Up within a twelve-month period.

You'd probably have to be completely uninitiated with Death Cab to go into the film expecting any real debauchery; Motley Crue they ain't, with highlights instead including squabbles over whether it's dorkier to like Woody Allen movies or The Lord of the Rings and an assertion by bassist Nick Harmer that they "put the punk in punctual", with an apparently unblemished record of turning up to venues earlier than necessary. The live footage shot at their homecoming show at The Showbox in Seattle sees them all wearing smart shirts, ties and trousers, looking more like a group of IT technicians than a rock band; a little digging around the special features reveals that to be as part of a tribute to The Office, but the fact that it doesn't strike you as especially jarring says a lot about just how unassuming the group really are.

That's why, then, it's perhaps not too surprising that guitarist and producer Chris Walla departed the fold this past weekend with no hint of fireworks, unless you count the meteor that lit up the skies halfway through their set at Rifflandia Festival in Victoria, BC; you can only assume they hadn't arranged for that. Instead - as the YouTube footage has it - Gibbard introduces the final song of the set as Walla's last ever, they plough through a typically-rousing rendition of 'Marching Bands of Manhattan', the clearly-emotional foursome embrace one last time, and suddenly I seem to have something in my eye.

Speculating on the precise reasons for Walla's decision to quit the band is likely pointless - my own assumption, for what it's worth, is that a man who's long had fingers in a variety of pies has simply decided that he can't continue committing to two-year tour cycles as he approaches forty - but there's certainly plenty of room to both mourn the demise of one of the most potent pairings in modern indie rock and consider the ramifications thereof, both for the future of the band and for Walla himself.

Perhaps unsurprisingly, given that he's taken his future into his own hands so readily, it's Walla's next steps that are the easiest to forecast. "There's not a lot of things in this world I feel like I'm really excellent at," he said in a 2008 interview with The Backstage Pass. "One of them is Scrabble, two is that I make a really good espresso drink, from years of working at Starbucks, and three is making mid-fi indie records." He's actually one of the most underrated producers currently working in that particular sphere, and perhaps one of the most underemployed; that's more to do, of course, to his obligations with Death Cab than any lack of offers. On the likes of Tegan and Sara's The Con and Sainthood, The Decemberists' Picaresque and The Crane Wife and The Lonely Forest's Arrows, there's the same little subtleties of approach; they all sound crisp, clean and invitingly warm, and the guitar sounds, in particular, are always captured in strikingly sympathetic form. His next move will likely be to find his Hall of Justice recording studio a permanent home as the proposals flood in. He'll also, of course, be able to dedicate more time to his label, Trans-, which already has an impressive track record; Now, Now's Threads and Sombear's Love You in the Dark have brought sharp indie rock and brooding synthpop to the imprint already.

Walla might also make another solo record, a follow-up to Field Manual, which dropped back in 2008 to a relative lack of fanfare; it probably wasn't helped by the fact that its release came just months before Death Cab's Narrow Stairs. There was some superb stuff on there, though, and whilst it probably serves as compelling evidence as to why Gibbard did the singing in Death Cab, Walla's shortcomings in front of the microphone were made up for by the complexity of the record's sonic textures and its unusually nuanced political commentary; opener 'Two-Fifty', for instance, predicted the near-collapse of the U.S. auto industry in 2009. Walla has form on that front, too; he's a dedicated enough supporter of the Democratic Party that television cameras were able to pick him out in the crowd during Obama's second-term victory speech in 2012.

For Death Cab, meanwhile, the picture is less clear. Their as-yet-untitled eighth studio album, which features Walla, is slated for release early next year; it was produced by Rich Costey, although suggestions that his displacement of Walla in that role might have led to the latter's departure are almost certainly unfounded. It looks as if Gibbard, Harmer and drummer Jason McGerr will continue to play live, meaning that the most pressing matter is deciding whether Walla's replacement will simply join them onstage or become a full-time working member of the band. It's hard to know which is likelier, but if they go for the former option, it's bound to hand Gibbard's critics some immediate ammunition; the perception in that camp is that, without a genuine creative equal in the band, Gibbard will be able to treat the group as a glorified solo vehicle.

As crucial as Walla was to the band, it's probably also true that the lazy critical reaction to the last Death Cab album, Codes and Keys, perhaps magnified his significance to an inaccurate degree, whilst also unfairly doing down Gibbard's contribution. The singer's marriage to Zooey Deschanel provided the context that framed the album, which was quickly dismissed as the band's 'happy record' on the basis of songs like 'Doors Unlocked and Open' and 'Stay Young, Go Dancing'. In fact, Codes and Keys wasn't necessarily happy; it was just better balanced than the rest of the band's allegedly 'depressing' catalogue to that point, with cuts like 'Codes and Keys' and 'Some Boys' hinting at residual nervousness and the likes of 'Unobstructed Views' and 'St. Peter's Cathedral' merely representing acceptance where once there was uncertainty. In actual fact, only five of the eleven tracks on the record could really be considered unabashed love songs, which is probably more restraint than I'd manage if I'd just gotten hitched to a beautiful, world-famous millionaire.

Gibbard and Deschanel have since divorced, though, and that was what you'd initially have assumed was set to dominate the discussion of the next record, whenever it came; the reaction on social media to the news of that split involved lots of perverse pleasure at the prospect of Gibbard writing better songs whilst depressed. Those observers were never likely to have been vindicated on album number eight - it's probably safe to say that late-thirties Gibbard, as a teetotal ultramarathon runner, has a more rounded view of life than the guy who turned out songs as bitterly vitriolic as 'Tiny Vessels' - but all the same, it's interesting - and gratifying - that, instead, the album will be talked about in terms of its status as Walla's swansong, rather than be pored over for evidence of post-divorce despondency.

Gibbard's lyrics have taken an undeniable shift towards the straightforward over the course of the past decade, sure; it was something that began with Plans, came to the fore on Narrow Stairs and was reinforced on Codes and Keys, as a man who had once dealt almost exclusively in metaphor started to cite Randy Newman as an influence with an entirely straight face. In actual fact, though, that's why the band's genuine fans will have been so surprised, and so disappointed, by last month's announcement; Death Cab have had perhaps the most natural and organic progression of any of the top-tier indie rock outfits. They started out in a small college town, steadily built up a regional, then national profile with relentless cross-country touring, and gradually made the transition from dead-end day jobs to career musicians, from clapped-out vans to comfortable tour buses, from sparsely-attended rock clubs to theatres, auditoriums and the occasional arena, too. Gibbard started the band in his early twenties, and turned thirty-eight last month; accordingly, we've heard him as a perennially drunken hopeless romantic barely out of university, as a twenty-something jaded, but no more sure of himself on The Photo Album, and as a man comfortable with letting his emotional guard down on Transatlanticism. It's not a surprise, by any means, that his darkest songs, on Narrow Stairs, coincided with a time of apparent loneliness and heavy drinking, or that his most positive writing came during something of a honeymoon period. He's always portrayed both his own, and the band's progressions totally honestly.

And that's why it's hard to know what's coming next; you'd have been forgiven for thinking that Death Cab were simply going to carry on forever, with Gibbard and Walla continuing to become more assured in their respective fields. The latter had shown similarly impressive growth to his bandmate ever since Transatlanticism - experimenting with digital recording on Plans, making Narrow Stairs viscerally raw by cutting the songs live, and then turning one-eighty on Codes and Keys, using the guitars only for colour and punctuation. Without him, the creative see-saw is going to swing dramatically in Gibbard's direction; LP number eight, which the frontman just recently described as "broken where it needs to be broken", will certainly put the seal on the Walla era, but it'll be the album after that one that defines what future - if any - Death Cab can have without him.