Which I read as "It's bad that people get into 5E and enjoy something other than my preferred style of play." The vast majority of people who play are not gaming geeks (being a gaming geek is not at all a bad thing). They don't give a fig about "innovative" or "clean" design, they just want to sit down with a game with others that they have fun playing.
I've introduced quite a few people to D&D, especially 5E, over the years and the vast majority simply enjoy the game for what it is. Since most of them are, and always will be, casual players, I personally think D&D's overall design and themes have a lot to do with it's popularity. Many other games target much more of a niche or enforce an overall style, D&D is more flexible than many of those games. There is no monopoly power to D&D, the TTRPG market is very open. Currently D&D dominates but as others have said there have absolutely been times when other games could have risen to dominance and they didn't. Shows like Critical Role certainly play a part in that, but there's nothing from stopping people streaming any game they choose.
In any case, one thing I do think happened that stifled diversity is the OGL. We can't go back in time to see what effect the OGL had, but it seems like the majority of creatives that want to do something in the same basic target niche as D&D have reinforced the brand by using the OGL instead of doing something new.
