To simplify things:
Club has sold 13000 season tickets and say they are as an example £300 each.
This means the club earned £3900000 in season tickets sales.
If the club moves and say 8000 of these people decide they can not go to where ever the new ground is then this reduces the amount City have earned by £2400000. This will then leave a £2400000 hole in the budget for the running of the club.
It doesn't matter when the club may get the cash, it's the fact that they won't have that cash is the issue.