The reality of the situation is this:
(i) Every club should have a designated penalty taker, and claiming that if someone earns a penalty then they should get to take it is ridiculous. It might be a luxury for a team that are miles ahead in a game, or absolutely flying generally, neither of which applies to United.
In the 90s, when Southampton or Newcastle were awarded a penalty, Shearer didn't allow David Ginola to take one because he'd earned it and has a big ego, and Le Tissier didn't let Francis Benali have a go because he's his mate. Shearer and Le Tissier took every penalty because they among the best penalty takers ever, and very reliable. You don't even allow anything other than your best free-kick takers to take free-kicks, and there is a far, far reduced chance of a goal in that situation, so you certainly don't muck about with penalties, which can be absolutely decisive.
(ii) That's leaving aside how weak it now makes Solksjaer look when he now has to pretend that this was all planned before the game. Yeah, we're going to have two designated penalty takers, and then let them decide on the pitch who wants to take it. That makes perfect sense. According to Solskjaer, it's good that Pogba took the penalty because he wanted to take it. Well, what happens if Rashford wants to take it as well? Which he probably did. What Solskjaer said in the post-match interview is specious, at best, the obvious reason being that he couldn't say that Pogba had effectively overruled him on the field of play, simply because he felt like it.
(iii) It's hard to say exactly what happened on the field of play, but it certainly seems that Pogba just decided that he was going to take the penalty, and that Rashford had been expecting to take it. It certainly didn't seem that it was definitely known that Pogba would be taking penalties. And, anyway, what sort of team swaps its penalty takers from one game to the next? Why would you take Rashford off penalties when he scored in the previous game? What sense does it make to have two penalty takers, when both of them are likely to be on the field when you're awarded a penalty? Are you going to have an on-field debate every time that you have a penalty?
It's pretty obvious that Rashford was the designated penalty taker, which is normal practice for every club to have one, and that Pogba simply overruled him because he thinks he's the big star, and then missed. I'm not a United fan, it's just obviously the case. If it's not the case, it's simply preposterous for a team to have two penalty takers, for neither of them to have seniority, and for them to potentially be in a position of debating before every penalty who will take it, depending on who 'feels like it'.