This is the major one. I work on a 10 year old rails app and it has got to the point where we are terrified of making any change that has the potential to affect areas outside of the visible git diff. It's easy enough to manually verify regular changes by looking at the code. But something like a library update is impossible to verify and we constantly end up with production issues because of something changing in a library that wasn't mentioned in the upgrade guide and would be impossible to have considered beforehand. But that a type system would catch.