This. Well, I’m not saying I agree he’d fire O’Brien but yes, Caserio would be the boss. On most teams the GM is the boss.
We’re all just speculating on all this, but what really needs to be considered is something none of us know: What does Caserio want? Would he rather be the #2 guy on a stable championship contender, or does he feel he’s accomplished all that he can here in NE and that he wants the challenge of being the #1 guy, albeit in with far less stable franchise? If he wants to be #1, is he willing to wait for Belichick to retire or does he want to strike while the iron is hot?
These are things which we simply don’t know and can’t know. I guess we will find out in about 9 months or so.