Doyle, who you mention, is a case in point. He's never going to be good enough to hold a place in City's team. On the other hand they could have received a fee for him after his loan at Cardiff.
The question is though:
- Does it help City to have to sell him a year before they actually did do (for what seems like an artificially low fee)?
- Does it help Doyle to have to move out to a likely worse club than he has done on less money a year earlier than he would have had to?
- Does it help SUFC not having had Doyle on loan last season as we wouldn't have done under your rules?
The answer to all those is no. Forcing players out at an arbitrary age doesn't help anyone. Heck, in Doyle's case, do City even pick him up in their academy in the first place, or is he needing to go to somewhere worse?
If a player is desperate to play, they will find a way to do so, someone choosing to move to a club where they know they are going to be a marginal player for more money doesn't have the right to cry about not having game time if the club buys in players who are clearly better than him. Either pick a better club for your career development, or stay at your current club where you know you are getting playing time.