Having excellent player 3 years and then getting a little profit from him. Not to mention that if things are going good for him and club, there is a chance that he signs new contract without clause.
Yeah, that sucks. If we are buying player then it is 10 years contract or nothing.
I see your point but in terms of United's image as a major club it would be quite damaging, and hurt the brand also.
It would be a bit like admitting we're a smaller club than City/Real/Barca/Bayern etc.. Like saying okay we'll sign you but there is another level to go up after us. It might be true of course but that doesn't mean we want to act like that.
A bit like when Arsenal started selling their best players to City, eventually people started to disregard Arsenal as one of the top clubs in England. As top clubs don't regularly have their players taken off them by a rival.