You are still in tactical mode.I think it is simple. One interchange a game for any player is all if it not for serious injury.
If you try to take a player off a second time and it not for serious injury he stays off for good.
That right there stops the coaches exploiting the bench for things it was not meant for. There cannot be mass rotations if you not give the luxury to exploit the bench for things it was not intended for.
It was not meant for rotations but before mass rotations was un-intended consequence of extending it to four there was never an issue with interchange that were no subs only. So I do not believe you need to strictly make them all four subs. I mean, you do not even need 4 subs and that would be a waste as then you got 3 or 4 players every round that virtually had no football at all on your list.
How can a coach make changes at 1/4 and half time if players can only come off once? Already is getting complicated.
The breaks are there to give the 18 a rest.
Yeah, normally you don't need 4, but I guess the concern is concussion plus an injury or 2 and bang 4 subs is needed to ensure it is 18 v 18.
Making them replacements is actually about fairness, go back to picking your best 18 and create a plan to win based on those 18 playing 100% game time.
Not the current 22 where about 4 players play 100%, and the rest are then load monitored to play 85-60% game time.