Actually I don't think united have played this badly, yes they are paying totally over the top for Sanchez but it makes a statement that they are able to pay more than anyone else even for a player so highly converted by one of their main rivals, who up until this transfer window was seen to be able to go toe to toe with them.
As for what does that do to Pogba and co it sends the message that untied are the ONLY team in England that can pay "the market rate" To me it seems they have created as many problems for us, simply by paying this much as themselves.
People seem to want to see salary and transfer fees in isolation, but that just isn't the case. If you want a graphic example look at the way the British record fee exploded in the 1979 the year started with Kevin Keegan's fee of £500,000 to Hamburg being a British Record and ended the year with Wolves(of all people!)paying Aston Villa £1,469,000 (largely on the money we had given them for Steve Daley! but that's another story!).