Yes, he will probably develop into a very good player, that is why we are talking about paying extreme amounts of money for him. At 45 mill, which was the number our discussion started at, he would be the 6th most expensive player in the history of Manchester United.
I don't understand why you keep repeating that Palace doesn't need cash, every single club in the world would want to get a fresh cash flow into their budgets. I agree that Palace do not have to sell him to prevent the club economy from collapsing, but selling AWB would probably allow them to double their transfer budget this season.
If you actually believe that AWB would rather want to play for Palace than United, he has absolutely no ambition and will never reach his potential. Of course he would like to move to one of the biggest clubs in the world, where he will have big chances of being our first-choice RB. His salary at United will probably be 10 times as high as well.