lnhammer and bondgwendabond say useful things about the problems with assuming YA and adult SF/fantasy publishing is a zero sum game in the first place. Because with the exception of a few lines like Starscape and Mirrorstone, YA SF/fantasy isn’t sold by imprints of adult lines, but mostly by completely separate imprints of completely separate publishers that think of themselves as YA imprints first, with SF/fantasy, mysteries, adventure stories, gossipy contemporary novels, problem novels, and all the rest sold side by side.
From a YA imprint’s point of view, that YA fantasy trilogy is competing not with the adult fantasy trilogy on the other side of the bookstore, but the bestselling Gossip Girls book and the award-winning animal story on the same shelf, as well as with other YA fantasy trilogies. A YA reader is debating between Stephanie Meyer and Judy Blume, not Scott Westerfeld and George R.R. Martin.
One could argue that in spite of this in the big-picture view adult sales are still dwindling because folks are shopping in the YA section, but I’m not sure I’m convinced it’s quite so cause and effect. I think–especially since there are differences in the types of stories being told even when you factor out the coming-of-age thing–that it may be more like adult SF/fantasy is for complicated reasons failing to connect with older readers as well as it used to, at the same time YA SF/fantasy is succeeding in connecting with younger ones (and some adults) in greater numbers than before–that these two things are happening for two mostly different reasons.
As lnhammer and bondgwendabond say, the two “genres” really do act mostly independently of each other, and there are only a few people who even could decide to focus on YA instead of adult SF/fantasy because it sells better.