Every United fan I know is always convinced that some player (either a signing they want or a current player who they want to see used differently) or a new manager is going to be enough to turn them into world-beaters. Then they get who they want and it makes fuck all difference.
The truth is that while of course they could be improved, they have some talented players (no-one I'd take in our starting XI on current form, but talent nonetheless) and their managers since SAF have been a mixed bag but not all total shit (Mourinho and Van Gaal have won plenty, and a look at Moyes at West Ham shows he's capable).
The issue they don't seem to want to address is that of mentality. It's all very well Rangnick saying they need to run more, but I don't think that's really the problem. It's not that the players aren't trying, it's that they're all pulling in separate directions, trying to be superstars (or to avoid being singled out for criticism) rather than trying to make the team as successful as possible. They can run all they want, but they're not running together.
It's a pervasive rot, the whole club needs gutting and rebuilding, moving away from the obsession with the past, cutting down/weeding out some enormous egos and instilling (rather than just talking about) some proper standards. But they can't see the problem, so it won't get addressed. I expect we'll get a good few years of schadenfreude out of watching them piss money away trying to find their saviour.
Probably too early to say, but I think in a few years we'll look back and say that Arsenal post-Wenger, for all their faults, recovered and rebuilt faster and better than United post-Ferguson.