The problem is Sane is better at everything than Mendy apart from Defending. But Mendy's defending hasn't been great. Sane is faster, a more accurate crosser in my opinion, has better ball control and is obviously more of a goal threat while staying just as wide and providing the same kind of width.As you suggest, it's extremely harsh on Mendy who is still very good all those things (apart from the goal threat) but it's hard to play both of them on the same side and Sane is the superior player, he just needs a more defensive player behind him.