i've recently had to deal with similar situation, accept the person that refactored my code didn't make such a big improvement. instead he went on vacation and a lot of things broke when we released the code. even though i've looked at all his check-ins, and they looked harmless, i was not able to foresee the problems we experienced when this code run in prod..

i'm conflicted because on one hand i don't want to be the code police that simply refuses any changes from others, but at the same time its hard to be responsible for a system when so many core updates are done by other team mates w/out proper testing.

i think the happy middle ground is: no such changes in common trunk. Do those in a branch, and only merge them in when benefits are clear, team is on board with all changes, you're ready to release to production, and will be around during the release.