I get the argument you're making but it really doesn't apply here. Son's committed a tactical foul but he's not used any unnecessary force and his challenge in isolation would almost certainly not cause an injury. The injury has come about because of some really unfortunate circumstances. Unless you're rewriting the rules and are outlawing tactical fouls for sporting reasons, rather than the health and safety of players, then that is never a red card.
Tackles like the one from Choudhury on Salah the other week or the one on Rashford yesterday, you have a point. In both of these instances there was no attempt to play the ball and they've not just tripped the opponent, they've used unnecessary force to bring them down. It's situations like these, where a tactical foul has been committed and put the opponent in danger by using far more force than needed to commit that foul, that red cards should be issued.