Diamond leaderboard is full of champion accounts with the lowest rating of 3720 rating.
How come people don't move up?
I don't rent anymore, unless testing a new card or one I don't have - but I am guessing there are plenty of accounts that have been milking the cheap rents over the last months.
Currently, leaderboard placement was based on the starting league in a new season but they did an update so that next season will be based on the highest rank achieved from the previous season.