Since no one puts a hand on Brady he could easily play for years. It's not a 'rough and tumble' game if you NEVER get taken down...
You are leaving out one very important FACT regarding Brady.
Just like Dan Marino, he gets rid of the ball very quickly.
Therefore, he does not get hit as often.
Mahomes, is an incredible talent. Maybe the best I have ever seen. However, when you are scrambling around making those incredible side armed passes you WILL GET HIT HARD. Just like Lamar Jackson, you will get injured. Scrambling QBs do not have long NFL careers.