while that's an example that I do see in the wild, not every function returns interface. but it doesn't really hurt my argument.

if I have a function that returns an interface I can grep who is calling it and probably digest what it is within a few moments and probably 2 or 3 files of code.

with java I find its so once you get through the factories, abstractions, and more, I have never found what I'm looking for in 2-3 files until I am more intimate with the code base.