I understand the point the OP is making, as there are going to be a lot of "boom or bust" situations in the league this season, but surely it's impossible to really even have this discussion until we have a little more clarity on player movement?
As someone has said, United could finish top or sixth or anywhere in between, realistically. Well, top and fourth, anyway, if you want to really be realistic. But if they get Cronaldo and Alcantara, then even Moyes couldn't possibly get that squad to finish lower than second. And if they lose Rooney and only bring in someone like Fellaini, then you could see it easily being worse. The same goes for double for Arsenal: if they really do splash the cash and buy a top striker and a DM, they could challenge for the title. If it's yet another case of raising expectations only to dash them by signing a 17 year-old French winger, then they could drop out of the top 4 altogether.
I just think this whole conversation is about 6 weeks early. Let's let the transfer dust settle just a bit before we start to rhapsodise about this team or that one falling off.