Had to think about what to post but here goes nothing.
I don't know how many of you remember early rgl, but back then we had makkabeus as the head admin. He was incredibly over reaching with his restrictions and often times would class lock ppl who had simply placed high in the division the season before, failed to find an invite team, and were looking to play the season on their main in advanced.
Now this happened to me multiple times. For over a year I had to play fuckass soldier seasons because I literally could not find an invite team and was not willing to make one. So instead I played soldier, lost a lot since frankly I am not good at all at soldier and didn't bother to practice (whoops.) But it still took my head out of the game and made me heavily resent the admin team and more importantly, heavily resent pretty much everything tf2 related.
My point in this is to say that class restrictions don't just prevent old players from returning, as millie said above, but also lock people off their classes for an extended period of time and, such as in my case, cause players to no longer wish to compete seriously.
Last thing I'll say is as someone who played during the esea era, I can sincerely tell you that the sandbagging was not that bad. Yes! sandbaggers won most seasons. But it didn't change the fact that the divisions were much more competitive and the general parity between mid to top open/intermediate was a lot better. Just removing class restrictions and letting people play their mains (outside of incredibly egregious examples) should be good enough to keep the divisions fun and stable.