We only talk about numbers but the length of the contract it's important as well. Even if keepers tend to last more in the elite, I think many here would be surprised to know at what age good keepers like Casillas, Valdes or Cech to name some started to lose their 'mojo' and that intouchable aura at their clubs. I don't mean being totally done but starting to decline.
I really think it's naive to consider a renewal for 5 years like something good for us at this stage of his career. One season is such a long time and many things can happen, so let alone five. There's no memory in football and heroes can turn into villains in a matter of months, and I think the moment De Gea starts reacting just half second late to shots we'll have another headache for years and a very expensive one. So 5 seasons on 400k (or even more) in my eyes is like a ticking bomb.
I could be more relaxed about it if he was showing progress in other areas at least, but he's still timid at leaving the goal line to win the aerial ball and anticipate the deep pass. He's actually worse at playing from the back since LVG and Frans Hoek left, at least I remember him being half decent which he isn't now. So happily throwing money based on how good he's been under the posts and the player of the season awards doesn't strike me as a very good decision.
There's also the domino effect, because if the keeper is earning outrageous numbers any good outfield player will be asking for similar money. This is why I can't buy the argument that giving De Gea 'whatever he wants' is cheaper than signing a new keeper. It could be in the short immediate term, but it keeps making the snowball bigger for the future.
The main problem I see is that without De Gea we'll need another keeper as I don't believe Romero is good enough to start here every week. If you see all the gaps we need to fill this summer only in the starting XI, thinking about a keeper and also a midfielder if Herrera leaves could be far too much. This is why I would try to convince him with 300k which is far too generous, and 3 years + 1 optional which gives some margin to re-evaluate after some time. If he doesn't take it then I would start finding a replacement.
There's the option in that case of selling him this window, probably with PSG being the most likely destination and get funds to sign a replacement. Or just keeping him for next season and leave the door open for an agreement, while we do a more intense research in the market for the following summer, hopefully with most of the needed signings being done already. Whatever happens it's not the end of the world, that's for sure.