Optics is the key one. As has been said elsewhere, the United hierarchy (Woodward and Judge, and then Murtough and Arnold) LOVE to see mega deals hyped up and try to make the club look like Real Madrid picking galacticos off a tree.
But in reality almost all our big deals we did massive favours for other clubs by taking their cast-offs and paying twice the market value in as short a time as possible. No one at the club has the intelligence or nous to do difficult transfer deals.
Casemiro perma benched at Real, if a club like City had gone in for him they'd probably have signed him for £40m rather than £70m. Mount a known sick note and not in the plans at Chelsea, double market value paid. Liverpool or Arsenal wouldnt have gone above £30m. Dortmund knew Sancho was never going to be a star and they could replace him easily, double market value paid, and an extra pat on the back for taking a year longer to knock off 10m euros. Hojlund, Amrabat, Varane, VDB, Fred, Maguire, AWB, Martial... all the same story.
Ronaldo, Ibrah, Falcao, Schweinsteiger, Sanchez... easy fecking peasy "big-name" deals to pull off just by overpaying.
And when we sit back and look at the big picture, our two best post-Fergie signings are still Herrera and Shaw. The only two who were scouted thoroughly, paid market value for and signed not-dependent on who was manager.