You’re completely right that nobody is stopping a PR that just ever so slightly decreases understandability of something. It may not even be obvious to the reviewer that it’s slightly less understandable overall… Doubly so if they’re just skimming due to the volume, and tripley so if they’re having agents do the most of the review, which lets the human reviewer’s guard down.

But before LLMs, it didn’t matter because on a good team you’d have a balance where maybe some PRs added complexity and some simplified it. Sometimes people would include a small related cleanup or reactor in a PR for code around what they were doing if they noticed complexity. That has become far less common, I think.

I know it’s not your point, but a 1% decrease 100 times is 0.99^100 = 0.366 which is a 63.4% decrease in understandability. Just throwing that out there