I think that's a bit unfair to the compatibilist position. Ultimately an account of free will should include a definition of the concept, and an explanation of how it figures into our intuitions about particular cases. In particular it should explain situations in which the given definition conflicts with our intuitions, and obviously determinism is a big part of that.

The incompatibilist argument is at heart very straightforward: free will is incompatible with determinism; determinism is true; therefore free will does not exist. When pressed on the second premise, the incompatibilist can further assert that indeterminism is incompatible with free will, and that determinism and indeterminism exhaust the possibilities (this is just an instance of the law of the excluded middle). Therefore regardless of which one of them holds, free will does not exist.

However, even this argument requires a definition of free will, in order to demonstrate its incompatibility with (in)determinism. It thus suffers from the same 'problem' that you allege compatibilism does: the compatibilist can simply say that the incompatibilist is defining the problem into existence.

I'm curious to know what you think an answer which didn't suffer from the problem you outline would look like.