But if we do expand the set under consideration then I'd suggest they do in fact have specific things in common. Large flying carnivorous reptiles. That won't cover all the various edge cases but I think it describes the vast majority of the popular usage of the term.

That definition would imply that sufficiently large flying dinosaurs qualify as dragons. And at least personally I think I agree with that conclusion. Dragons aren't purely fantasy, they're merely extinct (and never breathed fire IRL).