Sorry but hardly disagree with almost everything. By that logic every top goalkeeper should be born and raised straight from the academy of the top club, if not, he is not top club material. Neuer was 25 when he moved to Bayern, Alisson at 24 was not even in Europe and he moved to Liverpool at 26 etc.
I watch a lot of football and have watched ~95% of DDG and ~80-85% of Raya/Sanchez performances last seasons, and I would take the latter two over DDG on any given day, despite Sanchez currently being out of favor at Brighton. There is a strong reason why they were included in the last Spain NT squads and De Gea wasn't.
As for Sanchez's current situation, given previous situations Brighton already faced with both Trossard and Caicedo this season, I don't buy it completely that they just gave Steele a chance or that he is a better fit for De Zebri. I might be wrong, but I have a strong feeling there is something behind the scenes and that they are simply preparing for a life without him.
And yes, Brighton and Brentford might be "mid-table" clubs as you described, but they are also among the best-run clubs in the league.