It still may be hard to resolve when something is as simple as possible.

So modal realism (the idea that possible worlds exist concretely) has been highlighted a few times in this thread as an unparsimonious theory but Lewis has two responses to this:

1.) This is (at least mostly) quantitative unparsimony not qualitative (lots of stuff, not lots of types of stuff). It's unclear how bad quantitative unparsimony is. Specifically, Lewis argues that there is no difference between possible worlds and actual worlds (actuality is indexical) so he argues that he doesn't postulate two types of stuff (actuality and possibility) he just postulates a lot more of the stuff that we're already committed to. Of course, he may be committed to unicorns as well as goats (which the non-realist isn't) but then you can ask whether he's really committed to more fundamental stuff than we are.

2.) Lewis argues that his theory can explain things that no-one else can so even if his theory is less parsimonious, it gives rewards in return for that cost.

Now many people will argue that Lewis is wrong, perhaps on both counts but the point is that even with the case that's been used almost as a benchmark for unparsimonious philosophy in this thread, it's not as simple as "Lewis postulates two types of stuff when he doesn't need to, therefore, clearly his theory is not as simple as possible."