In watching the SB, Russ saw that a more mobile and much younger QB got squished. Time and time again.
Being a decade older than Mahomes, he still saw his future: One ACL away, one high-ankle away. You see what not playing in 2020 has done to JimmyG's stock. The odds only increase that it'll happen to Russ with his style of play far, far closer to the Murrays and Lamars than Brady.
The only realistic alternative PC has right now is to develop a scheme with more short routes, underneath YAC, etc. As well as strategies not just for checkdown but throwing the ball away. How many times have we all sat there yelling "RUSS THROW THE BALL! THROW THE BALL!"
And then see how it goes.
The worst that could happen is a losing season. Which neither of them want.
Getting rid of RW would probably Belichick PC's career. Keep in mind they're the same age. I think PC would rather go out as someone who has technically had a winning season for the past decade and only missed playoffs once, than someone under whom the season tallies are more like 4-12 than 12-4.