The 49ers are simply having to or trying to lean on Purdy more than they want to because the running game isn't working and there and theres no balance on offense.
49er RBs have 18, 16, and 15 carries the last 3 weeks while Purdy has thrown 27, 30, and 31 passes. That's a terrible run to pass ratio. In their 5 wins, the least they ran was 25 times, and the rest were 28+ times, and Purdy only threw 30+ passes once in those 5 games. Ideally, they want to run it 30-40 times and have Brock throw 20-25 times. They can't have him throwing way more than they're running, that's just not a recipe for success in the NFL, and it's not how the 49ers are built to play.
He's not a guy they can lean on like that. Not yet, anyway. Will he be at some point? Maybe, maybe not, but he's a 7th round pick who's learning as he goes on what he can and can't get away with in the NFL, and learning the hard way. It doesn't help when your vaunted defense, the one you count on to keep you in games, puts you behind the 8 ball by letting teams get leads early. That's not how this team is built. It's built to run the ball, play good defense, and get efficient QB play. When 2 of those things aren't happening, its gonna look like the last 3 weeks have looked.
When Deebo and Williams come back, I think we will see the offense we saw in the first 5 weeks. Say what you want about him needed an all-star cast around him to succeed, and maybe that's the case. Purdy isn't a Josh Allen Mahomes type that can put a team on his back, and that's OK, nobody expects him to be. It'd be nice if he could, but if he was that type of player he wouldn't have been a 7th round pick.
Will he ever be elite? I doubt it, but I still think he's good enough to win a SB with this team, but they have to be healthy and the defense has to get back to what it was. It doesn't matter who people say is the best team, or has the best roster. The best team doesn't always win the SB, any team can get got come playoff time. And the 49ers have shown the past few years they can bounce back, go on a run, and become dangerous at that time.
Call him mediocre, or terrible, or whatever you want, but he's shown he can play well in big games and on the road (before the 3 game skid, of course) and he does have 2 playoff wins under his belt (thanks to the complimentary football the 49ers are used to playing, it's not  an individual win). I don't believe he's regressing at all, he's just being asked to do more than he's capable of at this point in his young career, and it's not working out. When he can go back to just playing point guard as they say, the offense will be fine IMO.
The defense though...that's another problem they need to fix to even have a chance at a playoff run.  If they can fix the defense, it'll help Purdy and this team tremendously.