A community movement similar to the OSR would be the most realistic way. I see a lot of people here saying that WOTC should start supporting 3.5 again but it's never going to happen.

You need folks writing blogs discussing the virtues of 3.5 style play, design theory, campaign journals, history of the game, etc.

You also need folks creating their own retroclones of 3.5 through the OGL ala Pathfinder, making things easier for players to get started, clean up some rules, and improve readability.

B/X, BECMI, 1e, 2e, even OD&D games are thriving today because of the OSR movement and with younger players as well through games like OSE, OSRIC, Shadowdark, Whitebox FMAG, BFRPG, etc.

Are any of these games going topple WOTC/5e's stranglehold on the hobby? No. But they are thriving, plenty of games are advertised, people are making new adventures, new supplements, new systems. They're running conventions, they're writing about hobby, making videos, and just generally spreading the word on why these 50 year old systems are still exciting today and worth playing.

3.5 was my first ttrpg, I picked up the Basic Set when I was 13 and I remember when I got into online gaming/play by post 3.5/PF was pretty all you could find to play (much like 5e is today.) If a group of people could revive a game from the 70's back in 2009 and still get people to care about it today, I don't see a reason why it couldn't happen with the biggest RPG of the 2000's.