Unless remacs gives a clear advantage over emacs in terms of performance while still retaining compatibility people won't switch easily. Even then the switch should be quick and painless. (Neovim had those and so some people jumped ship).
There's a moral issue too in giving-up copyright to an entity : What if the future the FSF (RMS is not eternal) changes its policy to turn to something completely crazy with your code ? They will own all will own all your copyright-assigned code and you won't be able to do anything about it.
I mean : what if you were agree with GPL2 but not 3 and that now the copyright owner decides to move the code you've written to GPL3 ?
The FSF's copyright assignment contract has clauses that prevent them from doing such things. It's really a very fair contract.
This is as good a reason as any to try to replace Emacs. Maybe Remacs will succeed.
Though I suppose it'd be best to have proof of this claim. Not that I don't believe you, but this is the type of thing that turns newbies away from Emacs without giving it a shot.