This is United's problem. It only becomes a major problem if all the well supported teams suddenly become rubbish overnight. I don't recall the same nonsense when Liverpool were struggling (Souness years, Hodgson) or Spurs being bottom for a good few weeks for that matter. But, if Arsenal, Chelsea, Liverpool and (now) City were to fall down the same rabbit warren as United are in at the moment (a bit unlikely) then, of course, the "brand" would be affected. For me it's ok if another team becomes successful but - like all sports - your big names can't all fail collectively for there not to be a fall-out in terms of the marketing of the product.
I don't remember a major crisis when Blackburn won the league and they can hardly be described as a major draw (although that was pre-global brand days).