I wonder how much more happens only after you say "fuck it I'm doing this thing that we've desperately needed to do regardless of manager approval or sanctioned time."

When the whole dev team is in on it it's actually not too hard to bake in extra time for the unseen things by just everyone taking longer than they would if they were doing purely what the managers wanted. (Though ideally by doing some of the unseen things the total time for future work will become less than it would have because you don't drown yourself in tech debt by trying to do the PM's MVP only every time... It's nice to have a PM who understands that tradeoff.)