If you are going to have to break backward compatibility anyway, of course you are going to use that as a chance to change other things, fix some things that weren't broken but weren't great either, try for some nice to haves while you are at it.

Especially if you were predicting a migration across any compatibility break would take decades or more (or potentially forever live side-by-side), there's very pragmatic reason "rip the bandaid off" and to change everything all at once rather than make a series of far more painful "smaller" migrations that would each individually take nearly as long.