"Elite" and "greatness" are functions of time.
Wanting to give someone an elite label to prove a point doesn't make the point any more valid or the player any more talented.
If he is great, there should be no concern with waiting another year or two for him to prove it.
As for being Brady's in OT, I will quote a high school coach from my past...
"Don't blame the refs or a single play on losing the game. If you were that much better than your opponent the score should never have been close enough to matter."
The Chiefs had 4 quarters to effect the outcome and didn't. They lose because they lost. Ask the Saints if the one play prevented then from making the SB or if they should have won the game in the 4 quarters leading up to it .
Mahomes isn't "elite" yet. He's just played one season. Simple as that.