Mahomes wouldn't be near the QB he is without Andy Reid.He IS a system QB but there is a difference in being (solely) a benefactor of the system and being part of the system's success.
People act like every great QB was the only reason they were great when, in reality, it took players and a system to maximize that ability. Brady had security blankets underneath like Edelman and Welker, then had super talented TEs like Gronk and Jailnandez as well as one of the all-time best vertical threats, Randy Moss. Rodgers (most recently) had Davante Adams when he was MVP. Mahomes has had Reek and Kelce. Brees had Graham and Colston...and the list goes on.
You need players that have the ability to make the plays work, which SF obviously does, and a QB that can read the defense, make the right decisions, have have precise ball placement. That's EXACTLY what Purdy is doing right now and he deserves a lot of credit for that. There's a reason why Shanahan was so quick to Dump Lance and Garroppolo once it was clear Purdy was making his recovery. They knew they had a guy who could place the ball, make the reads, and understand what Shanahan was/is trying to do.
I said this earlier, but I think Shanahan is brilliant, even moreso than his father was. Mike really pioneered the zone blocking scheme that made Denver a running juggernaut, regardless of who their running back was. Kyle is amazing at devising plays where his targets run specific routes with the idea of having them open progressively throughout the protection cycle. Just like anything it doesn't always work, but go back and watch a lot of their plays. If you break down the play in a short 11-2/3 second window, there are outlets Purdy can hit if pressured. If he has longer, there are vertical plays downfield he can exploit. If he has all day or gets flushed out of the pocket (throwing on the run) guys are open or know exactly how to break off their route. Then you have the ultimate safety valve in McCaffrey who can run, block, and catch out of the backfield.
The 49er offense is the best offense in the league right now. They have excellent design, a good offensive line, talented skill players, and a QB who understands the offense. I don't think people understand how HARD it is to have a (good enough) QB and head coach/player on the same page. I almost guarantee you that Shanahan wakes up with massive boners every morning knowing he has a QB that understands and can execute his offense.
On an almost equally sad note, it royally pisses me off our offense was a FRACTION of what it should have been while Brock was here. Manning/CMC ran a total piece of **** offense and did little to fix the timing windows that were constant issues while he was here. That **** should be a crime.
With Jimmy G, he would greatly reduce the pass plays. I remember in the playoffs they had a few games where they would barely even throw the ball. With Purdy, Shanahan mentioned that he has to resist calling a pass play every play just to keep it somewhat balanced. This shows that Kyle has complete confidence in Purdy.