We'll see if enough users actually care about this new restriction to leave Twitter entirely. Personally, I doubt it.
My prediction is the rule will be temporary, in effect just long enough to hinder early-stage growth of these new social media alternatives.
Lots of feathers ruffled by Twitter banning promotion of competing social media
Anti-competitive business practices are highly effective. Look at Apple, they are experts at building a closed garden of proprietary hardware and software. Apple's house, Apple's rules. It works.