He was accurate, he didn't make mistakes, and he never seemed rattled. He was nifty on his feet. He's going to have a really good career with the Niners (darn it!).

Purdy was not the reason the Niners lost.
The Niners lost mainly because they faced a team more experienced on the biggest stage, and that plays slightly better in the biggest game. And it they had Mahomes. Just because you can go down a roster and compare it and say one is better than the other doesn't mean that's how it works out on the field.

But mainly the Niners lost because 1) their rookie Kicker missed a PAT (so KC could employ an entirely different approach, rather than having to take bigger risks). 2) McAffery fumbled when the Niners were pretty much assured a FG at the minimum, but the way they were easily moving the ball they more likely would have scored a TD. 3) One of the worst calls I've seen in a Superbowl by Shanahan, in passing for a 3 and 4 at about the Chiefs 10 yard line, with 1:52 to play, and the Chiefs with 2 time outs.

Not much to say about a missed PAT except it happened.
Not much to say about a fumble, that happens, too.
But, that call in the last 2 minutes made no sense. It was no better than 50/50 that the Niners succeed to make a 1st down whether running or passing in that super-compressed part of the field. If they ran, then the clock runs, and no matter if they don't make the first down, more time is taken off the clock and KC is forced to use one of their 2 time outs. Instead, by an incomplete pass, the Niners essentially gave KC a 3rd time out, and it wound up taking that plus the other 2 time outs for KC to win. If they had had only 1 time to work with, KC likely would have had to settle for a FG and a tie. That second time out is what bought them the ability to run one more play that resulted in the game winning TD, before they would be forced to kick a FG.

Really, really bad call strategically. Unwisely aggressive.