De Gea seems to be on a decline. Maguire is overrated. McTominay is average. And players like Pogba doesn't seem to want to be at the club, Martial blows hot and cold, James is so-so. They have A LOT of deadwoods in their squad - Shaw, Jones, Young, Fred, Matic, Pereira, Mata, Lingard etc. United almost have to buy an entire new squad to get back to challenging at the top.
United have thrown a lot of money in recent years and it's gotten them nowhere, not when people like Ed Woodward is running the club basically. Pochettino would be better off biding his time and wait for opportunities elsewhere. The Bayern job still not filled permanently, Tuchel and Guardiola could leave PSG and Man City respectively in the summer, Zidane is always a few bad results away from facing the sack at Real Madrid etc.