Yeah the main thing being the game changed away from Bills strength and into his weaknesses.
The WRs and passing game became more important than ever and let's be honest he hates QBs and can't draft or doesn't value WRs and still wants to win the old school way with a QB managing the game with defense and special teams where as the NFL wants shootouts and scoring and doesn't want defensive games and the rules are against it.
The Anti-Bill and Pro-Bill people can't just accept that Bill was the best ever in his prime and then at the end he sucked, we don't have to go back and re-create history to discredit him when he was great and then pretend he didn't cause his own demise with terrible drafting and coaching staff decisions, he became more about wanting to be with his friends and coach people he liked rather than to hire the best people for the job, if you don't get out at the right time a coach in every sport has that moment when the game passes them by and Bill had his.
Bill from 2001-2016 was a great coach who although had spotty drafts you could always say he put the team first, from 2017 questionable things where he clearly putting himself and his personal feelings before the team started to happen, the downfall started before Brady left that last Super Bowl was almost a miracle that team had some horrendous performances during that season at Jacksonville, Tennessee and Detroit that you almost never see from a team that wins a Super Bowl.
A Brady masterclass in KC and one last showing of Belichicks defensive fastball still having life against the Rams secured an unlikely championship for a team that was in decline that had no business winning.
Conclusion - Bill was wasn't carried by Brady for his career like the Anti-Bill people would tell you he was outstanding for a good 15 years, Bill also wasn't doomed by Mac Jones or Kraft forcing him to draft him like the Pro-Bill people will tell you he was doomed by his own bad decisions and the game passing him by which lasted way longer than a year or two it was going on a good 5/6 seasons and it was getting worse and worse every season and Brady staying would only be able to massage it to a fraudulent 10-11 wins, the days of this team winning big were done because of Bill and his bad drafting and his paranoia and refusing to hire anyone but friends, friends kids, his kids and people with connections to him.