"Inertia and the fact that [D&D] is also the game of choice for people who (otherwise) aren't aware of the TTRPG space" is the very thing I was talking about. Those things cause D&D to be the 900-lb gorilla. ANYTHING that gets the D&D label slapped on it instantly gets vastly more attention than anything that doesn't. Gamma World is a great example of what happens when the makers of D&D create a side game that doesn't have the D&D label. It disappears.
So, my actual thinking here is that I see the space a little bit differently than you do, mainly in that I don't really see the reality of the situation as especially static.
The whole environment more or less changed in the 2016 to 2019 stretch, which in some ways appeared to enforce the dominance of DND as a seismic slam of new players entered the space under the DND umbrella. But I think that what we're seeing is that not all of those players are sticking with the hobby, and the logistics of 5e have appeared to turn WOTC into a kind of pinata-- some of the new players quit or rarely play, essentially being "people who bought a player's handbook" while others are functionally to invested to stay within WOTC's sphere of influence without a whole lot of friction, and in that recent conversation with Mearls/Winniger there was some evidence shown that the audience is treating the 5.5/2024 thing as an edition split, with some ardently refusing the transition, and a smaller portion likely starting to shake off DND entirely (and by that I mean I've met and spoken to some of them.)
One of the major reasons for the OGL situation appears to be WOTC's identification that other parties were making too much money off of the previous license, like huge kickstarters for 5e material and the like, or the growing success of what were at the time OGL systems, like PF2e. To me this demonstrates that in aggregate, WOTC's 5e base that's been built up over the last few years, is watching the market hungrily reach out for material beyond what they're producing.
Overall, I see this as an ongoing process where these 'high-investment' 5e players introduced in the last decade have been 'coming-of-age' and jumping demographics and tastes, that doesn't mean that they're growing into anything
in particular (some are reaching for more roleplaying, more rules lite, some becoming more OSR-coded, others looking for an advanced high-customization style product, just like the rest of us all sorted in different directions in whatever relevant era) but it means that they're becoming a less tractable base and there's potential for more mass movement.
The problem is essentially that WOTC's dominance of the market relies
too heavily on the factors I mentioned, it relies too much on any given player who is jonesing for something else to have friends pressuring them to stick with the game as opposed to also jonesing for something else, it depends too much on the person being invested enough to buy a lot of product, and engaged enough to push the lifestyle-brand on socials or talk about the game on r/dndnext or r/onednd, but not enough to start looking into the other games that get mentioned, and it relies on the neither the product nor the purchaser changing in terms of the purchaser's desires.
That's a very, very, narrow win condition for keeping the rest of the industry out, and I think it's only getting worse as the high-investment players continue to develop into whatever kind of gamer they are and become less compatible or lose interest altogether-- and I'd attest that it's why the PF2e subreddit saw a spike in new subcribers every time WOTC ran a UA in the lead up to the 2024 edition (did you know that the software we used to track that went down in the API thing? I really wish I had the data for the aftermath of the new PHB's release.) The more stuff like that happens, the more likely it is that any given gamer you meet is going to tempt or pressure you (as a tablemate) outside of WOTC's space.
Edit: To circle back around, I think that 4e more likely had less uptake because it had a higher skill floor, rather than because it wasn't sufficiently DNDish-- so it couldn't pursue as strong a blue ocean strategy as 5e (which is where a lot of 4e's success came from! Myself included!) 5e is almost facing the opposite problem, where it doesn't hold up as well for advanced players, and many players go from needing a low floor to needing a high ceiling, a lot of the money spent above on 5e kickstarters is chasing that ceiling, not of power, but of... meat and potatoes? More to be invested in? Better support for whatever playstyle?