He's poor at 1v1s, but that's not the only aspect of defending for a full-back.
He's also a very good build-up full-back, who can invert into midfield well, has good physicality and athleticism, and has a mean shot on him. Decent passer and crosser too, and he's consistent, which is very important. I don't think he's good enough to start for a PL and CL winning side, but there's plenty of minutes for him as a solid, 7/10 every week type of player who can play to a good level on both sides, and United as a team are collectively not on that level yet where Dalot's minutes should be significantly reduced. You also need to consider that he has a really good mental profile, and loves the club, and he's been here for a long time now. Plus we got him for peanuts. These are not the type of players that you just sell because there's a chance to make a ~20m profit on them.
Him and Mazraoui are more than fine for now, and we can be patient until a world class young prospect appears on the market who's attainable for us.