A lot of people seem insecure about Mahomes having success, perhaps because it somehow harms Brady's reputation or takes media attention off of the Patriots?
Seriously, who cares. Brady's been to 9 Super Bowls, has won 6, and is objectively the greatest QB to ever play the game.
Mahomes would have to sustain success for another 2 decades, win a similar number of Super Bowls, and put up similar stats. The odds of him having that degree of success/longevity, and playing well for that long a period of time, is incredibly unlikely, which makes Brady's success and legacy so remarkable.
If Mahomes manages to be that good for that long, which is extremely unlikely, then he will be a deserving peer.
Brady's legacy isn't in question. No need to be insecure.