Possibility to be a spectacular failure for starters, and the obvious impact it will have on us the next time we come knocking for a new signing. Clubs already double their prices for us, this will just make it worse.
Not at all. Clubs have always known United can spend money, this transfer is not nearly exorbitant enough to take it into a different level. I don't think what you're talking about really happens to the degree you think it does. I'd imagine the conversation between United and a hypothetical selling club in future could go something like this:
Utd: "We think 50 million is fair for player X"
Selling club: "But you paid 120 million for Sancho"
Utd: "We value player X at 50 million for reasons A, B, C..."
Selling club: "Ah... fair enough"
Clubs will always seek to maximize their revenue, but there is a ceiling whereby they can't just ask for more money just because United have paid more for another player - it depends more on the competition for the specific player being targeted.