Also the issues around De Gea's distribution occur in the context of the rest of the team, which generally isn't that good at playing out from the back, progressing up the pitch under pressure and retaining possession.
ETH has tried to get around it this season by compromising and going long far more often than he'd ideally like, but he will be looking to change that as we go along. And that means upgrading some players.
And the issue for the De Gea is that once we get past the low hanging fruit that is a new RB and CM, the remaining players in our best eleven who aren't particularly good in that regard (namely Varane, Casemiro and Bruno) are all further down the list to be replaced than he is.
If you don't get to the point where you're replacing him in the team for someone who improves us in that regard, you're putting a ceiling on how much the team can improve. Because we'll still be carrying relative weaknesses in other positions too.