See this is the thing, when you are at the top its very easy to say that, United have never been away from it and are in the position of being Man United. without first Chelsea and then City, the premier league would be the Bundesliga. I'm not saying rubbish like City/Chelsea saved the league but its no coincidence that its the only league that isn't a cakewalk for the big dogs. Sure every so often Dortmund or Atletico will pull off a title but without in particular Chelsea, United would have complete Bayern like domination over English football. Purely of their own making but the PL would be a United win 7-8 of every 10.
With regards United, You can't see it right now but United are too big not to come back, you are already getting closer. Cities investment despite what you read on here has slowed down quite significantly in the past 3 or so years and will continue to stay at maybe 2 players per season. You guys can match that, what you were lacking is a clear vision in the boardroom. You've had 4 managers since Sir Alex with very different philosophies and as such money has been wasted. Moyes to LVG was a huge cultural and football change, LVG to Jose and Jose to Ole the same. This meant a lot of manager inherited players who didn't fit the systems/ethos they were bringing it.
Pep on the other hand inherited a lot of old players and dead wood like Navas, Zabba, Clichy, Yaya etc.. but the club had a clear vision of how it wanted to play at boardroom level (even if that vision was "Be Barcelona (right down to the fruit loop shirts)) so the transition from Mancini to Pellers to Pep was smooth despite Mancini and Pep being very different managers, and also had core components like KDB, Silva already there. It was no culture shock for David Silva to transition to tiki taka, it almost felt like the 3 years under Pellegrini was preperation for Pep once the Spaniards arrived in the board room. Even with our money it takes time to implement a clear philosophy and City are now at the end of essentially 8 years all rowing in the same direction, you guys haven't had that and if you don't stick with Ole you have to make sure the new manager has similar beliefs and football philosophy (as in plays fast, direct counter attacking football based goals coming as quick as possible after turn overs). Anything else and you'll be knocking yourself back another 3 years. You guys just need more football people at the higher up levels, City are very lucky they poached Txiki and Soriano (two guys who know football and have a clear vision on how it should be played, along with knowing the right manager for the job). Its arguable that this is the first season where Pep has had to rely on Pep's players now Kompany, Silva etc.. are gone and Aguero looks on the way out.
Basically United will be back when they find consistency in the higher ups instead of being so reactionary. Liverpool are a shining example. Eventually they got their shit together and it took 5 years of building along with serious investment but they did it. If they did, so will United.