Ancient dragons are obviously magical creatures. They can't exist at all without denying the laws of physics. Humans of course can do so.
I'm all for mundane developing abilities that go beyond what reality allows, but they are simply no longer mundane when that happens. I don't want high-level fighters to be dispel-able, I want them to be labeled as supernatural, because that's what they are.
I know "supernatural" is a technical term in 3e.
But for 5e, I think I prefer the term "preternatural" for an impossible effort. Its etymology literally means, "beyond nature".
By technical definition, "preternatural" effects would mean, magical but without depending on the Weave.
Hence, Martial powers and Dragon breathweapons are preternatural.
Here are some quick definitions of preternatural from various dictionaries. I feel the flavor of this term is spot on. It normally means natural yet extraordinary or inexplicable, but sometimes can still refer to magic.
preternatural
(American Heritage Dictionary)
• Surpassing what is normal or usual; extraordinary.
• Transcending the natural or material order; supernatural.
• Beyond or different from what is natural, or according to the regular course of things, but not clearly supernatural or miraculous;
• strange; inexplicable; extraordinary; uncommon; irregular; abnormal.
(Wiktionary com)
• Beyond or not conforming to what is natural or according to the regular course of things; strange.
• Synonyms: inexplicable, exceptional, extraordinary, abnormal, uncanny
• (dated) Having an existence outside of the natural world.
• Synonyms: paranormal, supernatural, unnatural
(Colins English Dictionary)
• beyond what is ordinarily found in nature; abnormal
• another word for supernatural