If you’re talking about middle players then I believe this already happens, though the bloke stays on until he starts losing effectiveness then returns to the field sometime after half time when he is fresh enough to play out the rest of the game.
But he also mightn’t come back on (unless he is a key player like Taumalolo) until they have wrung the juice out of the bloke he is replacing, assuming there isn’t much talent difference between the two. This way you get your ‘moneys worth’ out of each interchange.
So if Mannah goes off at 20 minutes, maybe he’s ready to come back on at half time, but doing so would mean he would have to play the entire second half, which would mean he would have to pace himself. So instead he waits until Matagi runs out of steam (maybe at about the 50th minute) and then comes on and goes nuts for another 15-20.