It just struck me, what if WotC revises the core books but the originals continue to sell like they do now? What happens if they outperform the new core books? What happens if they out preform them a lot?
When the new books are released, the old ones will be removed from production. Indeed, that will happen some time before 5.5e releases, in the hope that any stocks out there will be depleted before the release. So they shouldn't be in competition with each other.
As for the sales comparison: with one exception, each time they've put out a PHB it has sold fewer copies than its predecessor, so it wouldn't be unexpected with the move to 5.5e. That said, the one exception I mentioned is 5e, and it has been selling ridiculously well for years now, so we're in unprecedented times... so who knows?
But if 5.5e does basically tank, and we go back to 3e/4e sales numbers, then I rather suspect WotC's response will be to reduce output sharply and just let the game lie fallow for a number of years prior to (maybe) issuing a full-on 6e at some point. Because I no longer believe D&D sales numbers really has much to do with the quality of the game at all - it seems more to be down to something in the culture, and once this moment comes to an end I don't think
any edition would really do much better.
(That last, incidentally, is
not a comment on the quality of 5e, which is a very good game and a very good edition of the game. It's just that markets are bizarre and fickle things.)