Purdy isn't a bad QB. In the right system and with the right HC he is good enough to win including the Super Bowl. He's not elite yet but he could be. Look we all saw how Goff blossomed in the right system with the right HC. Purdy is no different. In fact, he's a fit for McVay's current system. What I see is a pocket QB (a bit undersized) who can go through his progressions and make good decisions. He's got decent arm strength with good accuracy.
This SB loss isn't on Purdy. It's on Shanahan. In some ways, he's like McVay in that he gets one dimensional of his own volition. Like McVay gets pass happy, Shanahan gets run happy. Had he trusted Purdy more in the red zone they might have won but he didn't. The difference between McVay and Shanahan is in their respective Super Bowls, McVay improved his playcalling and he won. In Shanahan's case, he didn't improve and some say it wasn't as good as his first Super Bowl.
Purdy has an NFL quality arm capable of making all the throws accurately. His strength is his ability to quickly go through his progressions and make good decisions. Paired with McVay and this Ram offense Purdy would have won that Super Bowl.
Unless Shanahan changes, Purdy will be just another Jimmy G. Make no mistake Purdy can be very dangerous in the right offensive scheme with a coach that trusts him. But look at Shanny's record with Jimmy G. and Trey Lance and you can see he's not good at developing young QB talent.