And he had a guy already in place so it didn't cost him any extra money.
Exactly.
It was a very unique set of circumstances to led to Jerry doing that and being "ok" with firing a coach midseason.
One, the HC got off to a disastrous start with a team that many viewed as being quite talented, and a guy who Jerry was absolutely in love with was already on the team to replace the aforementioned HC.
There's no other time in franchise history where that sort of scenario presented itself so that was a total anomaly.
Not firing McCarthy doesn't necessarily mean Jerry doesn't really want Belichick, it could just mean he doesn't want him enough to pay McCarthy for sitting at home while also having to negotiate a big time contract for Belichick at the same time.