Will Hunting's theory of upper class parents pulling their quarterback sons out of football at an early age due to concussion risks is a pretty good explanation on why the QB position has gone downhill. But I have another theory to add on.
Seems like this generation of players just doesn't care about the game or put in the time to 'love football' like previous generations did. Not saying it's a bad thing, but just what I've noticed with just about every swinging player (former or current) having his own podcast during the week, pimping brands on instagram etc. It's just the time we live in so it's whatever. A good example is that one Steelers receiver a few years back who made it a point to 'hit the stanky leg' on the opposing team's midfield logo and film it for TikTok every week. Despite teams saying they used it as motivation, he kept doing it because the social media street cred was that important to him. If I recall, he fumbled the ball one game too after a hard hit where the defender admitted he put a little 'extra' on it because of the disrespect. So it indirectly led to him playing worse.
Also, Micah Parsons inviting Stephen A on his podcast recently to argue about whether or not "Dak has it or not". Just a pure circus to build his brand. Back in previous generations, teams would rally internally to block out the noise. Nowadays, a lot of teams (especially Dallas) like to engage with the media circus for entertainment. Just shows, to me, that purely focusing on football is not a thing anymore