I think some people are getting carried away with the season we have had so far.
We currently have a 15 point lead but at the same time we have a had a nearly faultless campaign. This level is unsustainable over the long-term and we will drop more points as the season progresses. In addition, United have also improved from last season and are currently the second best side in the country. They have the spending power and the scattergun approach to keep spending until they bridge the gap. Anyone who thinks that won't work needs to give their head a wobble because it worked for us.
Allowing them to sign players like Sanchez is only going to help them bridge the gap to us and pull away from the chasing pack. It may not be enough to help them this season, but they will be in a much stronger position next summer with him in the side.