Before his spell at United, Jose Mourinho was almost universally regarded as a top 3 manager in world football at the very least. Pep & Lucho were complete unknowns in the sphere of management but still won epic trebles with Barca in their respective maiden seasons. The players make all the difference, it's as simple as that.
Money is the determining factor here, there is no way round it. All players on the planet are traded commodities and the richest clubs can pick & choose. The problems you are outlying for United are all short-term secondary issues. Of course it's very difficult for them to sign a player that Barca/Real/PSG and now even City and Pool are after. But they do not need to sign those players immediately. There are plenty of great players that the super elite clubs won't go after for obvious reasons. Look at City and how they built their team. Even though they were not a prestigious clubs (still aren't) they still signed players such as David Silva, Yaya Toure, Sergio Aguero, Kevin De Bruyne, Sterling etc etc etc.
Or look at Liverpool's current side (which most in here think is better than ours) None of Salah, Mane, Robertson, VVD, Firmino etc etc were coveted by Barca/Real/PSG. Yet they proved to be very good or even world class players.
One thing that many do not understand, is that the main advantage of being a rich football, is not so much what you can buy, it's the fact that you can afford to keep your best players. Players come and go all the time, but only the very richest clubs can hold onto those that prove good enough.
United will only be in real trouble if and when its sponsors reduce their endowments.