Really?! I've seen it several times this week. Maybe we mean something slightly different though.

My real world is looking at some function in an existing codebase that handles a nested data structure. At some leaf I can tell I'm handling a bunch of scalars by looking at the code (i.e. a loop or a reduction).

Somewhere far upstream that data was added to the structure. And somewhere downstream we might do something sensitive to the data type. But right here, it's just "a bunch of stuff." I rarely see the "bunch" being genuinely polymorphic, but I also just don't want or need to think about which collection it is to edit this code.... Except I do if I want to "just add item".

Moreover, it's not uncommon to want to optimize the upstream choice of container and need as little refactoring downstream as possible.

But yes, set and list are not generically interchangeable, of course. Maybe the stuff really needs to have mutables. Maybe it needs to allow repeats. Refactoring isn't just drop in replacement, and one needs to think of such issues.