You'll hardly find a server that's playing the game in a true Red vs Blue realistic modern warfare fashion.
That's because A3 and A2 as well, pretty much suck at PvP. It never has nor ever will be a decent PvP game. TB is cool, but so was ARMA-PR, and that croaked as well, because the engine is crap for it. Plus, ARMA always quickly turns into a key bind memorization metagame, that most people just won't put up with.
The vanilla game has always been set up for COOP. I don't have any problem finding plenty of populated COOP servers running stock settings. Nowhere near the popularity of the stupid rpgs or King of the Hill servers of course, that's where the 12 year olds are.
And obviously, any game that allows modding and any community that embraces it will have different life cycles than those that don't, those that release a "new" game every year. Can you imagine if RO2 or ARMA didn't release their SDK? Would they even exist after a year or two?
The fact is that there is a healthy population of players worldwide that like to play fps games that are in that semi-sim range, fans of "realism", however you want to define it. They will bounce around from game to game, or stick with one, but a new game like Squad or if FE ever sees the light of day, won't really create new players, it'll just steal from other games. I see it in Squad these days, yeah likely some new to that sort of gameplay, but mostly players and clans from A3/PR/Insurgency/RO2 etc.
The herd will stay about the same size with births = deaths, just the pastures will change.