all of those things apply to every sport to some degree though. out of x participants in a league, only y% are going to continue playing after a season, in CS, TF2, football, or chess. People play, realize they don't like the game enough to put time into it, and quit.
It being harder to get 6 people together isn't inherent to TF2, CS might be easier, but only because of a larger player base allowing people that no one likes to group together. There's only one BAD PERSON in tf2 but if there's like 10 in CS you can just make 2 teams (awful). People obviously get burned out on other games too, otherwise CS would be dominated by people from the 90s, but clearly people quit CS, and not everyone makes it past Open/IM in it either, otherwise invite would need to be substantially larger.
Get more people at the bottom, and the burnout rate wont matter because you'll still have more people filtering through to Open and invite. Short of changing how 6s works, you aren't going to make people who disliked 6s in steel like it in open. You could worry that 50% of tf2 players quit, while only 45% of CS players quit per season, but it wouldn't matter anyway.