Only activate the extra 2 subs after a medical assessment if you want to restrict their use to limit 'tactical' subbing.
But from a player welfare perspective nothing is worse than forcing players with a pcl injury like Bird on the weekend to play out a game because the sub is already activated. It's happening every round.
And when you get an a avalanche of injuries like the Coll game the result is compromised.
So you want to go back to the system pre about 1980?
No interchange, just reserves/ subs?