Sadly I think we have just been overtaken off of the pitch (as well as on it as well), for a long while we had by far the biggest club ground in England, so our profits from that certainly helped us to finance signings that other clubs couldn't, but as time has passed clubs like Arsenal, Chelsea (to a certain degree), City have all got either new grounds or improved their own grounds, so that the capacity differentials are not quite as much as it once was.
This combined with television revenues going through the roof, thus giving others far more spending power then ever before has resulted in us not being as dominant as we once were.
For example, no longer could we just purchase our rivals best striker as we did with Newcastle and Andy Cole, (can anyone imagine us purchasing say Aguero or Costa??) and then win the league.
Other clubs have got their act together more than we have off the pitch as well it seems, City have the group of clubs like New York and Melbourne etc, Liverpool have finally it seems stopped faffing about with trying to get a new stadium, and just increased their current one, Spurs have started (I think) building their new one, and on it goes.
We are still just plastering our "brand" over anything, and practically anything it seems, without any real foresight as to the development of the club as a whole.
There also doesn't appear to be any great deal of rush or any inclination to possibly upgrade our own stadium capacity wise, we seem to be happy as we are, ever since the takeover, that side of things has just been allowed to stay the same,with seemingly no ambitious plans for future or further development.
Once you stand still as a club, you always risk being over taken, and thus going backwards, and that is exactly what we have seen happen in the past few years, as the club, and especially the owners I think, relied on Sir Alex to continue our success, that they could just carry on as normal, but since he moved upstairs, we have shown no real ambitious plans off of the pitch.
It Is fine and dandy buying Pogba for a world record amount, but if the solid foundations as a club are not there, then it will not really matter who you buy, as things will tend to revert to type, and we are finding that out for ourselves at the moment.
Success (bar the odd cup victory here and there), true success, in terms of either a league or a Champions League victory, seems a long way off currently, and that is in part due to what is happening, or rather not happening off of the pitch.