Yeah, it comes up a lot. I think
@Umbran might have been in a few of those threads of late... I mean, I'm not complaining, nobody is really trying to be narrow-minded, but I think there are different levels of exposure to a wider variety of types of games, and it tends to be a point of view held by some who have stuck to a fairly traditional way of playing mostly D&D. Honestly, I'm sure there are a variety of backgrounds. Still, yes, there is a very strong contingent here and at RPG.net to an extent sometimes, who feel that systems like PbtA or other 'story games' are simply inherently niche games with very limited range. I think the standard argument goes that any whiff of meta-game takes them out of character. I don't want to be on risk of mischaracterizing though, I'm sure people can speak for themselves
I have often brought up a lot in various discussions about how
exceptionalism rarely holds up to scrutiny, and it's often a red flag of a person's prejudices and biases. It tends to say more about a person's views rather than the subject itself. This comes from my own academic experience, broadly working in ancient West Asian and Mediterranean cultures. When Egyptologists, for example, claim that "only in ancient Egypt do we see X phenomenon..." - often with the implicit judgment of Egyptian cultural superiority or uniqueness - it's not long before someone more familiar with other ancient Near Eatern/Mediterranean cultures (e.g., Hittite, Mycenae, Sumerian, Israelite, etc.) than the Egyptologist pipe in with "Well, actually..." (And a correction of such exceptionalism is sometimes followed-up with a moving of the goalposts.)
But I find it quite applicable to how some people talk about TTRPGs, particularly when it comes to D&D. I think that anyone claiming that D&D is more open to hacking and a DIY attitude than other games displays a certain degree of ignorance and lack of wider TTRPG awareness about other gaming communities and what goes on therein. IME, gigantic chunks of Discord discussions regarding these "niche" games is dedicated to hacking the game and demonstrating a tremendous degree of flexibility along the same veneer as D&D's. There is almost a double-standard that sees people hacking D&D as a sign of its flexibility and a lack of focus, but then regard people hacking non-D&D systems (e.g., Fate, PbtA, Cortex, BRP, Cypher, SW, etc.) as signs that they are more specialized, niche, and focused.
Stonetop, Masks, and Urban Shadows feel like completely different games despite all being PbtA. I think that calling them niche ignores the tremendous degree of flexibility and hackability that PbtA must possess as a system to even be hacked into such different games. That a system could produce such different "niche games" from a common resolution system and set of principles is not an easy feat. But the focus of PbtA is (1) genre emulation and (2) putting characters in situations of snowballing dramatic tension and conflict. I don't that is somehow more focused than D&D being designed for zero to hero fantasy adventure and fighting monsters in a series of tactical skirmish minigame encounters. I think that most (
but not all) games of D&D that didn't have semi-common combat encounters would result in a mutiny because that's what the characters are designed for and what players often want to experience!
I can tell you from my own discussions with other people who are not part of the hobby that any game that has cultivated the reptuation of people putting tremendous amount of personal investment into pre-play game prep, dungeon design, and decorating purchased terrain/miniatures is going to look like one helluva specialized, niche game regardless of whether theater-of-the-mind or dungeon crawls are part of the experience or not.
I don't find DnD (and anything built on the chassis like PF) to be all that flexible. In the olden days I tried using DnD to play different genres. At best it did okay, but never great. There are just things built into the system that don't work well for anything other than a traditional DnD game.
Spell casters become the be all and end of all of much of the game at even mid-level. For example, the spell system rapidly becomes: Spell X trumps situation Y, unless someone trumps spell X with spell Z. This is a poor fit for many (most?) types of stories.
Another area I find DnD poor at is long duration stories. Thanks to the steep power curve of the levelling system stories that go on for a while will either become a rather different story, at least in style*, or be rendered inert by the new powers that are made available. For example, I don't find it easy to run a mystery (say a long term mystery like the X-Files) across a wide range of levels thanks to all the divination spells that become available. Globe trotting adventures in which the journey plays a large part become kinda moot with teleportation.
*I have found Adventure Paths to be guilty of this.
I wish more games would be designed with horizontal progression in mind, particularly for spellcasters, such that you are not so much growing in terms of a vertical progression leveling power curve, but, rather, you expand horizontally in your versatility by virtue of having more tools at your disposal.