Side argument. @Flamestrike is right to assert that default 5e D&D is a game of superheroic characters fighting dragons at the upper levels. You're right to assert that default 5E D&D doesn't define D&D; "D&D" is a broad term with lots of latitude. Other editions of D&D are still D&D, houseruled D&D is still D&D. RAW 5e D&D shouldn't be viewed as the Animal Farm definition of D&D ("All D&Ds are equal, but....").
But I don't even agree with that (the part that I bolded).
What even is "default" D&D now?* If you play through the APs, then ... there is definitely some variation! And when I look at what "the kids" are playing, then the amount of variation grows exponentially.
I think the idea of "mythic hero" going on to beat up on dragons and gods is closer to an older idea of a Monty Haul style campaign than it is to what a lot of younger people do; which is more social, wuxia/anime, and has a lot of influences that go far beyond superheroes and beating up on dragons.
It's what you make of it; and superheores beating up on dragons is a very cribbed vision of the game. It can be, and is, so much more than that.
But yeah, I will strike down upon thee with great vengeance and furious anger those who attempt to poison and destroy my brothers by claiming a single definition of D&D and saying anyone else who plays a different way is just holding the d20 wrong ...
And you will know my name is Snarf Zagyg when I lay my vengeance upon thee.
*Then there is that whole side-issue that, in play, most people don't even bother once they get much past the "sweet spot."