I'm quite disappointed by the way this poll is shaping up. He's certainly at his all-time low in terms of his reputation among the fans (I'd like us to replace him as well by the way), but I can't see how he isn't one by any stretch.
He has made more than 400 appearances for the club, won United POTY 4 times, 5 times got into Premier League's Team of the Season... it shouldn't matter much, but even for the pure entertainment value — he has won MotD Save of the Season 5 times
He absolutely carried us through the worst period in our recent history, and his performance during that 2nd place season is still the best domestic season that I've seen by any keeper. He hasn't won much? Robson has only won 2 league titles with us — both as a squad player... Obviously, De Gea is not quite on Robson's level, but that argument simply doesn't work.
Is he good enough now? No. Should we replace him? Yes. Does it mean that all the good work that he has done for us over the years should be tarnished by his current mistakes? Surely not.