You would not swap him for anyone surely. Do you remember the Bartez, Tiabi and Carroll times?
Last season he was every bit as bad as Bartez and arguably Carroll were, and he pretty much single-handedly cost us top 4 with his mistakes in the last few months of the season. This season he's picked up his game a fair bit but he's still nowhere near as good as he once was.
De Gea was a fantastic goalkeeper, one of the greatest pure shot-stoppers of all time. But even at that time his all-round game wasn't all that good, with his ability under the high ball, distribution and his organisational skills (from what we could see) all being average at best. Because of that there was always a worry about how long he would stay at the top as athleticism and reflexes tend to be some of the earlier things to decline. I don't think anybody expected it to happen quite as early as it appears to have, but it was always a worry.
We're now in a position where we have to decide what to do going forward. Do we think these last 18 months is an outlier and De Gea will get back to his best? Do we think he won't get back to his best but he will at least be able to maintain this seasons form which has generally been an acceptable level? Or do we think this will be as good as he'll be going forward, and he'll slowly decline as he ages over the few seasons? And while we're making those decisions we have to factor in that we have a very promising young keeper on our books who is already arguably outperforming De Gea during this time. Everybody will have their opinion, but it's impossible to know what the right option is for sure.