You believe you are being rational? I expect you know how ratings are calculated. What was Mahomes rating in the first half? How many off the mark throws did he make then that but for luck did not tank his overall rating with interceptions?
Mahomes stepped into the same system Alex Smith ran last year. Alex Smith, who KC fans seemed to believe is awful as they booed for Mahomes to replace him, threw for 4000 yards and a 104 rating in 15 games (4270 if you project to 16 games). The Chiefs went from 10-6 (division winner) and a 1 and done in the wildcard card round to 12-4 and 2 and done in the AFC championship (both losses in KC before a friendly crowd). Poor Alex Smith had a 116 rating and 2 TDs to no INTs in that loss. Mahomes had an 85 rating and 0 TDs against the Colts in KC. Not exactly a world beating performance, or a harbinger of the promised one arriving when every win is life or death.
You likely were not alive when the Chiefs won a title in 1970. That makes you want to believe the Chiefs have found their Excalibur in Mahomes to do what hasn't been done for almost 50 years. Rejoice, for we have found our redeemer (and ignore that what happened this year is only incrementally better than what Alex Smith did last year).
It is irrational to compare Mahomes to Brady, or to call Brady a system QB at all. System QBs do not break every post-season record known to man, and there is no system that allows a QB to fire laser shots to move the chains repeatedly on 3rd and 10 on a hostile field in a championship game with the game on the line. The ball was in Mahomes hand, at home, and he did not win the game with a receiving corps objectively way more talented than the Pats have. His crapping away the first half is not a testament to a better supporting cast or coaching - it is on him.
The simple answer is stop by next year if the Chiefs don't come off the rails, in the event the Chiefs don't come unglued as many teams do after these losses. We'll see if Mahomes is a repeat MVP, or something less. I wish Mahomes the best, but unless he brings it year in and year out for more than 1 glorious season, he is just 1 more non-Brady QB in the NFL wondering what it feels like to hold the Lombardi trophy.