Look, if bitcoin could be destroyed, simply by a relatively large amount of miners deciding to shut down, how resilient would it be?
What's the effective difference, between Bitmain forking and Bitmain simply saying "we're no longer interested in mining bitcoin, see ya'll" and shutting their mine down, by literally pulling the plug? I don't see any. It's a drop in hashrate which sucks, because blocktime increases, but that's it.
Nobody would wet their pants, simply because one player in the field doesn't want to play anymore and I don't see how this is any different.