Geno is an interesting case, but Seattle fans think he regressed this year, and there was talk of benching him. You have to do it for more than a season. Consider how many one year wonders there have been. For instance, Case Keenum. His numbers blew away Geno's,
I don't think there's a single thing a coach can do to prevent bad habits from a guy like Mac. He throws off his back foot, gets scared, has a substandard arm. You would think in the offseason there'd be special emphasis on proper mechanics, but that's the QBs responsibility to make sure he is throwing with the right platform. A QB coach might help, but by the time you're in your 3rd year, you know what proper mechanics look like. You've had time to study them and employ them. But they are beyond Mac's capabilities.
I know we disagree in our sense of Mac since we've been through it the entire year. But I disagree with your assessment that Mac devolved as he year went on. I saw Mac's problems in the second game against Miami and I wrote about it here:
Key Players [Mac Jones/others] Isn't/Aren't Ready to Play
IMO, it was predictable he would devolve.
The rookie Mac that people remember? He was a mirage. He was scouted by NFL coaches, and they figured him out. He had no Act2. This is a common story. People go back and look at his rookie year as evidence, but we always see QBs have success for a year. Then the NFL takes away what they do best (throw over the middle). Once coaches figured out he couldn't throw an out pass, they clogged the middle. They figured out he was jittery and panicked, so they threw the kitchen sink at him. He had no way to combat that.