Saying that l.pop(k) has time complexity O(n-k) implies that popping something at position 5 from the end has bounded (amortized) time cost regardless of the length of the list l, ie even if we let the list grow arbitrarily.

It's a stronger claim than just saying O(n), because in the latter case you wouldn't be able to conclude that popping something 5 from the end has bounded time as the list grows.

Worst case can mean two things. For fixed n, worst list content and worst k, which gives you the less fine-grained O(n). For fixed n and fixed k, worst list content, which gives you the fine-grained O(n - k).

Edit: Another example of that is the complexity of convolutional filtering, which is O(n min(log n, k)) for a signal of length n and a kernel of size k.

Big O notation has nothing to do with worst or best case. f = O(g) simply means that f is asymptotically bounded by a multiple of g.

Counterpoint, I don't particularly have that expectation

And then there's G. H. Hardy, famous number theorist, who wrote an "Apology" for mathematics in the sense of defending the study of number theory for its own sake because it's beautiful.

Which one were you referring to?

It is one in the same! The apology is, of course, defending the beauty of the pursuit but at the same time the writing acknowledges the (alleged) utter lack of practicality.