The whole interchange & sub rules are actually the problem! Teams will always argue there should be more interchange players - from 2 to 3, then to 4 & so on all the way up to 20. Then the sub player was only for replacing concussed players, the poor sub has now become another interchange player! The whole thing is a rort, a rubbish arms race that benefits nobody! The only way out is to reset & start from scratch ...
I give teams 2 interchange players & 1 sub who can only be used to replace an injured player who is out of the game & automatically misses next week too. Cap interchanges at 4 per quarter. Injuries will happen - suck it up!
If teams run out of legs then they might have to stop the forward presses with zoning & endlessly running up & down the ground. Some smart team might actually rest players on the ground, deep in the forward half or even deep in defence. Teams might actually retain a structure with forwards, defenders & a small number of followers who have 4 interchanges between them per quarter to spread the load. Players can only be so fit, but players & coaches can be smarter & not trigger another arms race.
It is coaches who cause this, maybe coaches can fix it!