I think you know the answer to that question.
I think the related and better question is why is ETH seemingly determined to keep him? My guess is he is either saying the right things in public to limit the distraction but planning for an alternative in the off-season or he is for a limited budget in the off-season that will only allow him to upgrade in other, higher priority areas (like forward, winger, and additional bodies in midfield).
Because there's no way that he can actually believe De Gea is an above average goalie for the system he wants to play. In many ways, it's poetic that he and Maguire were the ones who made the most glaring mistakes. Both have skillsets that were much more valued 5-10 years ago and whose liabilities are amplified in the current era. For better or for worse, most big, successful clubs will require their back 5 to be comfortable on the ball and capable of playing out of the back. Neither of them will ever be even average at that let alone excel at that.
Word is that De Gea is close to agreeing to a new contract (with significantly lower pay) which isn't ideal BUT...if you surround him with defenders who can more often than not bail him out of his idiotic passes, then you can limit his weakness significantly. Or in other words getting rid of Maguire probably makes De Gea better if not an ideal solution...as long as you replace Maguire with a center back that has a better skillset for your tactics.