"We still have the Sea Venom, the FJ-3 Fury, the Sea Vixen (DH110), and the T-45 to count (forgetting the various Sea Vampires, T-33s and their ilk), even if we allow that the F/A-18 is more than an adaptation."
I'll just about let the Sea Venom pass (and to some extent other early straight wing jets, where landing speeds were less of an issue). The Sea Vixen was a nasty brute and more to the point was not really a derivative insofar as the land-based DH.110 never got very far in development. The FJ series diverged from the F-86 in the process of being made useful for carrier ops, and the FJ-3 bore only an outline resemblance to any F-86.
Historical experience is only a partial guide to the feasibility or advisability of the Seaphoon... but anyone who tells you that it's a low-risk, low-effort development is blowing smoke up your jetpipe. Look at the way that Dave C's wing has grown!