This is a common problem on all games which lead to question on why a system was never set up to account for it.
ie - IF so many Established tribes (%) say they want a server wipe then one is initiated say in a week.
Established as in active and been on the server for a month or 2 or......
Then it would happen.
Something like that and yes it is rough and would need to be hashed out the exact method....