It's not possible for United to prudent? You mean they have contractual agreements to spend a lot of money each season?
No, they've got ongoing contractual commitments to the likes of Pogba (in all probability), Ibrahimovic and the Granny Shagger which will propel them to the top of the salary charts as a club, I imagine. A policy of relative prudence in the latter years under Taggart was much easier to contain because they weren't recruiting top tier players, which creates enormous upwards pressure on overall wages, both directly and indirectly.
There's also the vastly different English footballing arena of recent (and future) years to consider too, in stark contrast to those final years under Taggart. In that sense, there's a degree of shit or bust about this on behalf of the Glazers, not that I think they're mistaken in following that path. They've got little or no choice imo, if they want united to remain relevant, in a material sense. The new TV deal, as well as our entry into the arena eight years ago, has completely changed the footballing landscape in this country, and they have to respond to that robustly and aggressively, or wither on the vine, Liverpool-style. They are now not only threatened by City, but other emerging clubs like West Ham and Spurs. Their golden era of supremacy is over, and they must know that, the question is, to what degree will they emerge at the other end, as a dominant force. Only by continuing to spend prodigiously will the Glazers give themselves a chance of providing the success on the field that would properly slake their lust for hard cash, given their wider, overarching financial commitments.