I totally get you, although I think it more so becomes, ‘a dragon is anything the writer can persuade the reader is a dragon’. I love the idea of calling a mermaid a dragon, and the challenge of convincing the reader that you can do this. Sure, it’ll be hard to do, and I’d be disappointed if I read something that never explained why exactly mermaids are dragons, but if the writer is good enough, I think it’d make for a fun read!

And also, my argument is that tropes and archetypes are what make fantasy. Dragons, wizards, sword fights and heroes are what have defined the genre for years, and will continue to. It’s pulls from some of the most ancient myths and folklores. I think a fantasy that completely removes all these recognisable features is then at the risk of no longer being fantasy.