They should change it to a percentage of sales. That would benefit them quite the same, while also not exposing some devs to financial risks.
If the only games you offer for download are making money per download, there is not much argument. They get their cent for the game, the player pays whatever the game did cost, and so on. (Plus additional costs for keeping track, that are unneccsary if they would just have kept it at sales income.)
But charging for installs is disconnecting the money the game makes from the costs. Sure, they have that threshold, but imagine a company that barely breaks even, but with 200k revenue per year. Maybe free to play with premium version. The premium gets the money. But Unity will charge for the installs of the ftp, not for the amount of money the company made.