I find it interesting that many folks seem to think that if there's a mass exodus away from D&D, whether by consumer boycotting, or 3PP no longer willing to hitch themselves to their bandwagon, that this will be bad for the hobby.
On one hand, I can see it, because by no longer having just one game system that the vast majority of gamers will buy, there will potentially be many. This means 3PP either have to create multi-system compatible products, or hope that some other system becomes the golden goose so they can concentrate their effort on that system.
But on the other hand, I see diversity of the TTRPG as a good thing from the player perspective (if not for the 3PP). I'm a believer in "system matters". I've heard many folks want to shoe-horn every genre and theme possible to run in 5e game mechanics. I've often wondered if these same people have ever played something other than 5e. But by using d20 OGL mechanics, you very much limit the style and feel of play. I think that when people discover new game mechanics and worlds/genres not touched by 5e rule systems, it will open their eyes even more.
While it's true that there may be less content available because there may be less 3PP producing it for your chosen system, there IS more to gaming than that. We used to play back in the day where there effectively was no 3PP and you couldn't even hop on the internet to find inspiration or ideas from others.
I also believe that the 3PP can evolve to become more world, setting and adventure focused, rather than game mechanic or stat focused. In other words, less focused on extending game system rules, including things like classes or character options or even new rules. Many world settings don't really need a lot of game mechanic specific info (except maybe for modern or sci-fi settings that's very equipment focused and the game system is very detailed about that). I think a lot of groups already do a lot of house rules and experimental home brews and even have fun doing it. I think what's really challenging for a Game Master is adventures and the worlds to play in.
The other fear I think, is people think if D&D crumbles, then there will no longer be a "poster child" for mass media to talk about. And that if this happens, then there will no longer be an effective recruitment tool for new gamers. But I think TTRPG has reached a critical mass where it's already entered the popular culture and as long as people play it, it will continue to be something new generations will look at. Heck, how many gamers here have introduced their children into the hobby? If Stranger Things had chosen say, Runequest, would that have bumped its popularity? Or would people have gone, "what in the heck game were they playing?". I'd argue that as popular as Stranger Things was, people would have looked up what Runequest is (look how the obscure song 'Running Up That Hill' got to #8 in 2022, even though it only got to #30 in 1985!!)