I agree. If he surprises his sceptics and scores a ton of goals between now and the end of the season United will want to keep him. However, instead of being able to get him for a fair fee Burnley will probably ask for two to three times that.
If Weghorst smashes it at Old Trafford he will want to stay, we will want him to stay and Burnley will know they can't keep him.
With two years left on his deal at the end of the season, Burnley will see sense in cashing in. However, I doubt the Burnley directors will want to let him go for the kind of cheap fee Besiktas had an option for. Burnley will probably point out how much we paid for Van Persie, when he was of a similar age and with a similar amount of time left on his Arsenal contract, and say inflation is much higher now than it was in 2012. It could get messy.
I think it prices him out of a move completely. At a higher price it becomes a comparison with other available strikers. Let's say Kane becomes available, who is objectively a better and more complete player.
We're not flush with cash at the moment, and in order to be a more attractive option than Kane for example, his performances would have to be up there with Van Persie's, which realistically isn't going to happen. At the £10M mark he could perform reasonably well, and be affordable as a cheap addition to Kane.
Even as a backup, at that higher price, you would have to consider if he's a better long term option than say Adam Hlozek or any number of young forwards who haven't quite broken through yet. He could of course perform well beyond expectations and be worth that, but the impact he would need to make to get a move at that price, at his age, seems unrealistic.
I think this means it is a loan and done kind of deal.