There's no benefit in De Gea staying if he is 100% on leaving for one more season, it would be much better to get something from the sale and get our new no.1 between the sticks as soon as possible.
Only difference is this - this season, we still have to strengthen other areas as much as possible - CB, RB, CM, ST. Add in a GK, and that is a lot of money. We can retain a WC goalkeeper who we know is reliable to help us move forward as much as possible, and look to strengthen it in the next window, where a GK will pretty much be the main priority.
Ofc, this calls into the equation the fact of whether or not DDG will stay professional for the year that is remaining. But I think he will. While he has not committed himself to the club, he has not tried to force through a transfer too.
But then again, we can probably get in Cech if DDG leaves this season. Cech will be top class for atleast 3 seasons, imo. And probably for less than 25 Million too, if Chelsea will sell to a divisional rival. It's a quandary.