I wouldn't say they're writing about the fall of the world so much as they are expressing their distaste of the state of the world in general - it's definitely not a new thing for artists to take a political stance or to be dissatisfied with society, but it does seem to be an ever-increasing trend these days, especially among those who never usually expressed a strong opinion either way in their work.
Considering the current state of things, I'm not too surprised, but I do fear that artists have started using everyone else's agenda as a way to get their own stuff out there. This isn't new, either, but now it's become so commonplace that it's generally accepted.
To elaborate a bit: As much as I love the Smashing Pumpkins, I'm a little worried about their upcoming album, since it seems to be yet another "America is going down the tubes lolz war lolz global warming lolz Bush sux" thing. Although the few songs I have heard are surprisingly good, Corgan's previous albums were mostly about failed relationships and hating Jesus or something. Until he got with Zwan and wrote a bunch of happy songs about Jesus. Anyway, I'm glad he's doing something different (which will probably end up being completely awesome), but it's still weird that he's putting out a whole album dedicated to... well...
this.