My experience with a medium-size but long timespan project was the two things you mentioned a bit dismissively are actually pretty massive time sucks. People are terrible at stubbing, and refactoring without types is painful. The vast majority of my Ruby experience is within Rails, so maybe that's a contributing factor?

Having types not line up at various boundaries (DB/API/reading from a file) is already a pain, but Ruby made it worse by having that bad data pass through many layers until it actually blows up somewhere far removed from the issue. I worked on very real bugs where e.g. a corner case lead to a date being deserialized as a string and then because the last few characters were numeric interpreted as a number so when treated as a date resolved as millis since epoch (or something similarly crazy). It took ~1 of those bugs for me to be convinced that I had no interest in dealing with those kinds of problems, and adding a 'Date' type means it fails in exactly the right place immediately and is a 2 second fix.

I agree with you though - it'd be a silly test to write, so how do you get the correctness/robustness without either types or tests that look like they're effectively validating types?