Yeah but see, the thing is the Crane Wife wasn't prog. Like, only the tiniest part of it was. I mean, it had two long multi-part songs, but one of those ("the Crane Wife") was really just good-old-fashioned Decemberists folky story-telling spread over three songs (which were at opposite ends of the album), and if we're going to start calling a tune "prog" because it developes a theme over multiple parts then we'd better be prepared to start calling Beethoven symphonies prog. As for "the Island", sure, that was proggy, but that's one track out of ten on the album. Which was otherwise a pretty typical Decemberists album, made with major-label money and everything that buys.
So see, at this stage I really don't think anyone can seriously claim that the Decemberists are a prog band.