Belichick would have retained Brady until he couldn’t stand up any more if not for the fact that he has an entire organization to run and had to account for the likelihood that age would catch up to Brady like everyone else.
Belichick never didn’t want Brady, but he had to consider the ramifications of a long term deal to a guy who was past the age ANYONE had ever been able to play the position at.
The compromise was the same thing the Saints did with Brees and Brady was offended. Hindsight isn’t available to know where a 43 year old will be in 3 years, nor is precedent.
Ultimately given the way the team performed with him in the second half of 2019, and the cap position staying on top has put them in, him leaving ended up best for Patriots. They could not have rebuilt and put a strong team together this quickly if they also paid Brady 75-90 million over those 3 years.
2020 would have been a substantial step back from 2019; it had to be because they couldn’t afford to keep their own or sign anyone new of note even without Brady, and 75% of the Free Agent influx in the rebuild of 2021 was paid for with the money they didn’t pay Brady in 20 and 21. They could have added a couple guys last year but only a small fraction of what they did (40% of the snaps on both sides of the ball were played by additions last year). So they would have suffered through 2 mediocre seasons and not be starting a rebuild around a 45 year old QB.