There is many reasons why I would leave forum.
First and the most obvious is death.
Then ban. If I would get banned, no way that I would be ban abuser and would start with new account.
Real life stuff. Maybe I won't have enough time for Bitcointalk because of family, work or other interests which will become more important than Bitcoin. Or maybe I simply will get bored with crypto and it won't give joy for me.
KYC - no way that I would do it here.
Another thing which wasn't mentioned is change of ownership. If new owners of Bitcointalk would be anti-Bitcoin and turn this forum into something like Bitcoin.com, there wouldn't be any reason for me to stay here. Even if signature campaigns will remain, there is more important things than it.
Great example is Cryptotalk forum, that was once advertised here in signatures. Due to lack of bounty campaign (last bounty campaign on that forum ran more than half year ago) a number of users left that forum. After forum owners reduced the amount of post reward, great amount of user left it. Only spammers and those who have high ranked accounts and use forum for referral link promotion are left there.
I completely forgot about this forum. Does it still exists? Without bounties it was difficult to imagine different faith for this forum.