His blunders towards the end of the season probably made us realise he didn't deserve the salary of a number one goalkeeper.
This in itself would be a problem.
Nothing De Gea did at the end of the season was different to what he did at the start of the season, or indeed at various points across the last several seasons. Absolutely nobody, including Ten Hag, should have only just realised the extent of the De Gea problem based on his blunders in the last few games. Hell, blunders aren't even the main reason he needs to be replaced. He's been here for 545 games,
nobody should be changing their opinion on him based on the last handful of those.
Because that implies that if De Gea had had that bad patch a few games later at the start of
next season instead of the end of this one, we'd have had the almost immediate realisation that we actually should have replaced the guy we just handed a 200k a week contract to.
Replacing De Gea this summer is the right outcome, but if we arrive at that outcome from the process described then we'll have been rather lucky.