I'm not sure they're entirely safe. Mahomes is going to be great for a very long time even if he has to play from the pocket more as he ages. It seems nearly impossible when you look at the enormity of Brady's regular season and postseasons numbers but any one of the league's top young QBs (Mahomes, Burrow, Allen, Herbert, etc.) could go on a run of 40+ and 5000+ seasons with the 17 game schedule. And with the ever expanding postseason who knows how many shots these guys will have at blowing up their postseason numbers too.