The numbers don't lie:
Garnacho = 30 games, 4 goals, 1 assist
No matter how many seasons he stays at Manchester United, he won't be able to score a very high number of goals. That's why it's better for Manchester United to sell him and sell other players who are underperforming and sign players who will perform well, like Victor Osimhen for example.
That is unquestionable to me too. The problem isn't deciding if they want Osimhen or not, of course they would love to have him. The problem is being able to pay for him, because he may want a lot and that is going to be an issue for them.
Surely United is a rich club, and has rich owners, but that doesn't mean that they can pay unlimited amount of money to players neither. If he is bought for 75 million, which is the release clause, he will be paid or at least will request a 20+ million payment at least, could be as close as 30 million. Imagine a scenario where he demands 30, and United agrees, that means on a 4 year deal, that will be 120 million, so the total would come to 200 million. United is rich, but that would make them strain their finances.