The situation with De Gea reminds me of the situation with Rooney when Val Gaal was in charge. He divided opinion in the same way. Fans would see Rooney miles off the top scorers list and claim we should upgrade. But then he'd have games where he'd play at a high level again and people would claim he's still our best attacker. And across all of that was the idea that people oughtn't criticise Rooney because he was a club legend and it was disrespectful.
LVG clearly had faith in Rooney or he wouldn't have played him, but there's a big gap between "good enough" and the best available. But I think LVG knew that given his club status, his respect from his teammates and his ability to turn up on the big occasion, getting rid of him would have caused such a rift that it wasn't worth doing.
With Rooney, he stuck around until Jose arrived, at which point he declined so far that it wasn't debatable any more. Indeed one of the best things Jose did at the club was to handle that situation, and he did it by simply playing Rooney and letting his football, sadly, do all the talking. No-one seriously disputed that Rooney was behind Ibra after the start to that season, and everyone knew it was time for him to leave by the end.
My guess is that DDG will continue at the club beyond this season, not because he's the best there is, there are better keepers out there for sure. Rather because he's good enough that he's not a liability, and as a result the furore of getting rid of him would outweigh the gains. I don't think DDG will go until either he decides to leave, or his form drops so low that no-one can disagree that its time to move on.