“Poor for three seasons” is a bit of hyperbole. Poor by his previous standard, sure. Poor, objectively, at times for three season, of course. But De Gea has not been consistently poor, relative to most other keepers, for the last three seasons.
In fact, he’s had a very good season in 20/21 but still short of his 5 (maybe it was 4) club POY performances.
The better argument is that De Gea is heading downward — he is, objectively, not at the peak he reached 3-4 years ago — and we should sell him while he’s still a quality keeper. But as it stands today, Dave > Dean.