Hussar
Legend
I'm not sure that's true recently. After all, if what you are saying is true, then well, where are all these massive increases in players coming from? Because, let's be honest, there hasn't been massive increases in the audiences for other RPG's. Not like there has been for D&D. What evidence do you have that D&D is not retaining the players? I mean, you mention Baldur's Gate 3 "at the time"? What time would that be? Three months ago? BG3 has only been out for less than a year. I doubt we're seeing any "fall out" yet at all.5E D&D is great at what it does: provide a simple, easy, clean fun game to casually play with a group of friends. It gives the Younger Gamers what they want: "Some fun time to imagine killing a dragon!"
D&D does have a huge influx of new gamers. All the time. Baldur's Gate three caused a big fall out. Many video gamers loved playing BG3 by clicking buttons and then wanted to make the jump to playing real D&D, for example.
D&D, and RPGs as a whole, are not big on retention: many people when young play D&D for a couple years...at most. Then they drop it hard. Years and years later someone might mention D&D and they will say "oh yea, I played that once" and they will never play again.
Players that do stick with 5E for a time often become...bored...restless...unstatsifyed or worse. The game play is always the same (as it was made that way). And this is doubly so as most games follow the same "houserules" and "Gentleman's Agreement" as every other game with no verity at all.
I find most such players jump at a chance to play an Old School or OSR games after all of the above.
Let's not forget that the massive growth in D&D predates BG3 by about five or six years. It also predates things like Critical Role by a couple of years as well.
See, your experience has been players will jump at OSR. My experience is that I couldn't pry 5e out of my player's cold, dead hands. Getting them to try anything other than D&D is a massive uphill battle.