I always thought if we won a trophy finished in the CL the club we would piss off the most isn't United but Arsenal. If we finish 3rd I can only begin to think how pissed they would be, and rightly so. That said, I'm sure we've got plenty of stick from gunners about us spending loads of money, how Arsenal are the beacon of financial prudence and stability yet still managing to get into the CL every year.. Yet now the only option they have left is to spend themselves. They can have their principles or they can have success, I'm afraid they can't have both. Stick by those principles and next year they may drop out of the top 4 all together (I still don't think Liverpool or Spurs would pass them personally, but we're already achingly close to taking 3rd, and even if we don't the sheer fact that we were 2 games away from doing so won't sit well at all).
Same rules apply to everyone else really. They can spend money or they can fall behind us, because we will succeed. But the fans can't have it both ways. If United finished below us in the league without spending you could put your house on the next thing the fans would be calling for.