To be fair, if you're in an indie rock band, it's a bit rich to expect mainstream popularity when your entire identity is based around differentiating yourself from the mainstream and possibly not even wanting to be popular. If your band was doing very straight ahead mainstream rock (or the kind of rock that used to be mainstream) and not calling yourself indie, then that would be a fair point about how this type of band now struggles to reach the success it once had. But indie bands, by definition, have never enjoyed giant mainstream popularity. There was a period when indie bands were regarded as hip and trendy, which most of them no longer are, but as Droste himself said, even in that era they weren't actually popular. The last time artsy, intellectual rock was the most popular music style was Paul Simon's heyday in the mid '80s.
I think Droste, like Simon, has equal or more skills as a pop songwriter than a rock artist anyway. Grizzly Bear can barely be considered rock, they're more of a psychedelic folk-pop band. Two Weeks is a legit pop song, albeit in a very old school style, but a style that continues to have some influence on chart pop even now. So if he just rebrands himself as a pop songwriter the way Rostam did, he'll do fine. He doesn't even need to shift his musical style, just stop calling himself rock, which was never a good description for his music anyway.
Rock isn't actually dead but "rock" that calls itself rock has been dead for decades. First it was rebranded as "alternative", then "indie" and now it appears in many forms. Some so-called hip hop now is actually rock, some pop is rock, some EDM is rock. Indie is still mostly rock, but no one identifies as rock anymore, even people who blatantly are making rock music call themselves other things that are less unfashionable. People under 20 don't even know what "rock" means, let alone relate to the idea of "rock," even if they like plenty of rock music that's marketed under other names.
Droste's problem is that he keeps calling himself a rock act.