Mac Jones had a very good rookie year. Then McDaniels left and Belichick thought his genius, combined with a special teams coordinator, turned failed head coach (Though it was the Giants, so who knows if he still can't do the job?) and defensive coordinator, turned failed head coach could run the offense, merely because one of them is literally a rocket scientist.
As sophisticated as NFL offenses have become, they evidently are not rocket science and I don't know they realized how far they broke Jones before Bill O'Brien came back to town by overthinking it.
All that said, I think we'll find that Drake Maye's starts under Josh McDaniels will be a better body of work than Mac Jones' under McDaniels' tutelage