De Gea is so old that it's time to part ways with Manchester United and have to be replaced with younger players.
He is not even close to the age that Edwin van der Sar was before he retired from Manchester United, and was an active goalkeeper in many important games. De Gea is not leaving the club because of his age alone, but because Manchester United see him as a player they no longer need.
I'm a little confused what the management of Manchester United are thinking when they need a good goalkeeper but don't want to spend more money
They are trying to do business, but they have to be careful in this business because another club may just decide to hijack the deal. The already lost Kim Min-jae to Bayern Munich. If they loose Onana, it will mean that they do not have a good team that negotiates this deals for them.