As you say, the trouble with that attitude, if that was the case, is there was still a chance Forest could equalise and take the game to ET. They won the ball back and had one last attempt at putting the ball into our box after the incident - just look what happened v Fulham on Saturday, and in our mad finish v Wolves.
So VAR's decision not to intervene on a blatant red card could have ended up effecting the result. To say nothing of the next few opponents of Forest who the player will now get to face.
So they really shouldn't opt not to bother doing their job just because it's late on in the game. There could have been repercussions in the remaining time, and as it is it still allows them to pick him in upcoming games that he should have been banned for.