The rules have constantly changed in favour of easier passing. Offensive schemes have also evolved and adapted. 65% completion isn't what it used to be. Whites are still doing great at qb. Brees was lighting things up at 41 and Brady has had plenty of great games, at 43 years old. That's insane and wouldn't happen 20 years ago. Rodgers is 37 and one of the best.
If 40 year old White guys can dominate the league there's no evidence that plenty more of younger ones couldn't dominate, they are just getting less chances and many aren't in ideal situations like Sam Darnold
Mahomes lucked out into a great situation. Alex Smith was written off a bust and did great on the same offense. They were slowly building up a great team and he was replaced, he probably could've won with the same team. Yes Mahomes threw for 4000 yards last year, 2000 came after the catch. The year before 5000, 2452 came after the catch . It's not necessarily a knock on talent but a 41 year old Brees gets criticism for doing the same exact thing, getting many yards after the catch, but his last full season the Saints ranked in the middle of the league for yac, he still passed for 4000 yards. Yet a few weeks ago before his injury many were speculating that Brees was washed up, while Mahomes is considered the best.
Rodgers has the highest passer rating in the league right now
Brees, Josh Allen, Tannehill both have higher qbr ratings than Watson, and Carr is only slightly behind
There's literally only a handful of qbs doing poorly. Like 90 percent of the league has around a mid 60% completion ratio so I don't see how this supports you're argument
Btw it's hard to say if Prescott would have thrown for 5k. I think with covid and a lack of training camps/ preseason defenses were playing worse than usual league wide to start the season