Mahomes had a truly remarkable season, and earned his MVP.
With that said, I wouldn't be too quick to the trigger on any career labels. Nobody saw 2008 for Brady after 2007. Gannon and Esiason had MVP seasons, but are not in the Hall of Fame. If health, personnel and personal drive are there, then maybe he is the future. Those are really big if's.
In professional sports, especially a contact sport like football, I wouldn't be quick to look past the season and the ability to put a collection of great years together. If you need a cautionary tale on the uber kid who comes out of nowhere to shock the world, see local product
Mark "The Bird" Fidrych. 22 year old All Star and Rookie of the Year with a 19-9 record, #2 in Cy Young voting, 2.34 ERA. Next year tore his knee, done in five seasons having never repeated that success.
What makes Brady the greatest is greatness over time. Others will grab the great years, but fortune tellers trying to call the future have repeatedly failed in calling his end, let alone the identity of those who will come to replace him. I like Mahomes, and I wish him well because he does seem to be a decent kid and is fun to watch, but I wouldn't be so quick to anoint him the future of anything just yet.