There has been 3 (going on 4) seasons of throwing money at the problem, almost just because United have lots of money. And along with this there has been far too much focus on attempts at short-term, quick fixes, of which the mega-spending is a symptom.
If United had started out 3.5 years ago (i.e. post Fergie) with the clear long-term aim - as the top main priority - of rebuilding a dynasty piece by piece, then by now they'd be more than half-way there. But instead they've tried to take money-fuelled short-cuts, allowing insufficient time for player development (e.g. Zaha and Keane as previous cases in point, Shaw as an ongoing example), too eager to buy the next shiny new toy and discard last year's model, too focused on 'big names'.
To re-build a dynasty you need patience and continuity, not constant chopping and changing of players. You need players who are young enough to be there for many years. And you need players who come because they really, really want to ... and not mainly because they've been lured by mind-boggling mega-wages. But above all you need a manager who focuses on the long-term, including youth development.
I don't see Mourinho as the right man for this dynasty rebuilding job. United don't need a manager who scans the horizon looking for the next £80m/£90m player to sign.