Purdy played ok BUT the argument against these non-elite QBs is that in order to win a Super Bowl, you still have to be elite in big moments. That's what Big Ben, Eli, Flacco, and Foles were in the Super Bowl. Purdy had a chance to get TDs twice at the end but couldn't do it and that's what separates those Giant-Killer QBs from the ones who go home empty-handed.
Same criticism we have of Dak. If he's not elite then fine, but can he be elite in big moments like that? So far, he hasn't shown that clutch ability in the playoffs. With so many great QBs in the AFC, you're going to eventually play Mahomes or another MVP-caliber QB in the Super Bowl so your QB play has to be great in that game to match that level from the opposing offense. The argument that you can win a Super Bowl with an average QB and great defense doesn't apply in today's league like it did 20 years ago with Trent Dilfer and Brad Johnson winning.