>Has D&D been used for a greater breadth than that? Of course.
So you finally-
>But that doesn't mean that D&D is implicitly an adaptable system or a good basis for other types of games.
For fuck's sake. You are making a pretty simple and basic mistake, again and again and again, and not listening to not just what I'm saying, but what you are saying as well.
D&D's "default" is essentially kitchensink (a phrase that means everything thrown in together) fantasy that also includes sci-fi and horror elements. It is a gigantic system, with very few games coming even close to its size. While at first glance this may look like a single, sprawling setting or a singular type of game, it's actually a wide collection of interconnecting themes and playstyles that can be transitioned between. But, if that's not what the group wants, they can instead focus on one of these themes and playstyles, which include low fantasy, horror, and so on. How can half your brain understand this, but your other half doesn't?
>RPG's are innately adaptable,
To different degrees, some moreso than others. D&D isn't a Generic System, but it's a large system with modular components built around a simple core mechanic designed explicitly to be added or removed at the DM's discretion. Compared to games that are actually built around a single focused, specific genre, it's really easy to tell the difference, and yet you have avoided this point again and again just to keep harping about how D&D can't do what it's always done, by your own admission.
Hell, even if we just take your statement of RPG's are innately adaptable, it doesn't really matter if it's more or less adaptable than others. By your own admission you agree that D&D is adaptable.
And yet, half your brain is still trying to argue.
D&D is well suited for more than just High Fantasy. This is an undeniable fact you agree with, but you're trying to say that what it's best at is what it's limited to, and that's flat out wrong.