> I'm saying that if I've abstracted out the filesystem into an interface the distinction of asserting against whether a method was called on that interface and asserting a side-effect is somewhat trivial.

But now you're enforcing a degree of coupling between your filesystem interface and your code. If you refactor the filesytem interface such that the overall logic hasn't changed, but the parameter order for one of the methods is updated, then you essentially double the amount of work you have to do (update the code and update the tests). If you just test the logic and not the interface, then you only have to update the tests that pertain to the logic itself.

You should read up on the concept of functional core/imperative shell.