Maybe it was not defined well.
You could hire whoever you like, but the affect of not suitable staff (including coach) should be massive.
In addition, currently coaches are mainly for training. They should affect game related issues as well.
For example - the randomness affect at game-shape of players. Taking really bad desicions during games.
A player making an "italian strike" on court (as the coach does not know how to hadnle him), etc.
As a suggestion, this itself isn't bad. I think gameshape being affected by the trainer level is something that makes sense in game and provides a new choice. I think instead that there should be a separation of "trainer" and "coach" into two separate staff roles, with coaches having some specialties or coaching abilities that can influence how players perform. There was some small discussion on this in another thread somewhere that I don't really care to search for at the moment.
The part of it I wholly disagree with is having it target teams in the highest division disproportionally. I am trying to find a way to figure out how to label your perpetual cries to punish those in the top leagues as anything other than trying to give yourself an advantage to move up. Game mechanics should be essentially the same in league I or V, for the most part. Things like salary floors are an exception because the magnitude of money that can be earned by tanking in a top league dwarfs a lower league, but there's no reason to ruin what appears to be a quite reasonable suggestion by adding unnecessary imbalance to it.