Instead of buying a goalkeeper, strengthening their other areas or youth talents might be better.
The point is very simple to explain.
DeGea's contract expires in June 2020, if it does not renew in this 8 months on July 2020 1st, he can leave Manchester for free.
Juventus currently has a great goalkeeper but De Gea's potential is undoubtedly greater, so if it is not an economical suicide (if you can sell the current goalkeeper for 35-50 million) it is an operation to be done.
If Manchester does not renew with him, it cannot do anything.
(As with Pogba, time in Manchester is about to expire, he still has a 22-month contract and after he can leave for free, and "usually" a player will not renew on his last year.)