What would that actually achieve, save making things slightly more complicated?
Serious question, not sarcasm. Like, what's the actual goal, mechanically, of doing that?
The reason they've moved it out of being a Bonus Action is the virtually all "Bonus Action" attacks/shoves/etc. are being eliminated. In general they seem to want to make it so Bonus Actions are basically very rarely or perhaps even never "offensive". By doing this they make the game more straightforward, and they make character building less about finding a way to use your Bonus Action to do extra damage, which unfortunately, right now, it is.
As far as I can see, making it a Bonus Action again for some PCs would merely make the game slightly more complicated for those PCs, without in any way nerfing TWF in general, because people serious about it are likely to pick up the Fighting Style, either as a Fighting Style, or as a Feat.