I mean, this is the argument for having good integration tests.
At some point someone has to figure out if the new code will break a system; if you don't have good integration tests you're basically left eyeballing the changes, and sure eyeballing changes can work fine in small teams, but at some point you need good tests.
I did some work on a ranking system last year, and by definition there's no way to roll that out incrementally, because, well, it is the central component deciding what thing to do/show, and you have to change the world at once, there is literally no other option. So you need good ways of evaluating these wide reaching changes.
"If you liked it then you shoulda put a test on it" :)
