Putting nice things in core should be done slow & careful ... and in most cases, preferably not at all.

But to do something nice in core, that will - oh by the way - break every theme out there, sounds like a good idea only if it makes sense to push a large portion of the existing theme-inventory out of Extend.

Is that a goal looking for a solution? Weed a large raft of under-maintained themes out of Extend?

If so, doing it with a core-mod is a misuse of core; and a lackadaisical way to address issues in Extend.