Mostly because refs back then were totally cool with allowing teams to just kick them off the park

Pep was way ahead of the curve on this one.

I think this is one way managers such as Pep, Arteta & even Klopp partially have been ahead of the curve, by building squads where a single player can play multiple positions. This allows better coverage of injuries and better rotation. Still not sufficient but assuages the effect somewhat.

Manchester United have been too focussed on putting all the eggs into one basket with huge $$$ signings since Pogba and ETH is partially at fault, buying players like Mount who can just play in one position (which is already, more or less, taken by Fernandes) and average dosh like Anthony.

Pretty sick deal they managed to pull, capped at 140k/week which was probably the highest earning players salary 15 years ago but now the best players are on like 2.5x that amount.