Science has a better definition of average than football fandom IMO. Over there, average is just the middle 2/3 of people in a group. They would just say an average starting QB is a QB you'd rank anywhere from about 8th to 24th and all average really means is a guy is neither elite or awful. That definition makes things so much simpler. Take the following questions:
Is Darnold an average QB? (Translation: would you rank him between 8th and 24th?) Absolutely.
Is it a reasonable expectation to be able to win a Superbowl with an average QB? (Translation: do QBs ranked 8th to 24th win SBs?) If we just count Darnold, Hurts, Foles, Flacco, Broncos Peyton, and Eli twice as the the only average QBs to win recent SBs, 1/3 of the last 20 SBs have been won by average QBs. Yeah, it seems very reasonable to think a good team has a chance with or without an elite QB.
Is Darnold better than Mayfield? Maybe. I dunno. They're both average.
Is Darnold better than Hurts? Maybe. I dunno. They're both average.
Is Darnold better than Kyler Murray? Yeah, that dude sucks.
But maybe that's the problem. It's too simple. You can't argue as much with stuff like that.