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Agreeing to a fully guaranteed, market-setting contract for a guy that hadn't played in two years.
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Moving forward with the contract after he was credibly accused of multiple instances of sexual assault.
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When he started and has continued to play like absolute dogwater.
Really, any of those would have been it, but combined they've set a standard for ineptitude that has exceeded even previous Browns-related expecations.
You can be an asshole as long as you're winning. When you stop winning, you're just an asshole.
Belichick is, famously, an asshole. But, he also has six Super Bowl rings and is a no-brainer Hall of Famer. However, he also lucked into maybe the greatest quarterback ever for literally every one of those SBs. Consequently, it's not surprising he can't replicate the same level of success post-Brady.
You look at guys like McDaniels, Patricia, BOB, etc. and it becomes obvious why his coaching tree is gnarly. Those guys are trying to impersonate Belichick without Belichick's track record of success, or the tools that built that track record.
If I were an owner looking to hire a coach, the last guy I would hire would be one from Belichick's coaching tree. It's not that his guys are bad coaches. I mean, they are, but there are plenty of bad coaches out there. It's that they're universally assholes.