TwoSix
Everyone's literal second-favorite poster
I would argue that if your characters have advanced to the point where they're fighting gargantuan dragons, they've proved themselves to be "mythic by demonstration". It's literally revealed itself during play.But what if we just don't agree with either premise?
Many people are aware that there are already superhero RPGs out there, and if they wanted to play the Avengers, they would play one of those RPGs, not D&D.
And there are, in fact, quite a few people who aren't playing "mythic heroes." At the very least, I have no idea if any particular character is going to be "mythic" or not when I start- that's revealed through play, and choices, and all sorts of factors.
If I know going in that it's already set up to be a mythic hero, that kind of takes the fun out of playing. Might as well just read some book about a mythic hero.
If the argument is that characters shouldn't be able to take on gargantuan dragons outside of narrative considerations like raising an army, then I would simply limit the level the characters can obtain; or make all dragons near demigod level potency.