That San fran defense is the same one that game up a bunch of points to the Raiders. And also gave up a 44 points to the best offense they played all season in the Chiefs. When they go up against the Cowboys offense who is capable of scoring in the 30-40 point range, or potentially the Eagles, Bills or Chiefs, if they somehow make it to the Super Bowl, then that will be a bigger test for their defense than anything they have mostly have to face during their winning streak.
Defensive coordinators know the weapons which makes Purdy the most comfortable, and there will eventually be one who is going to find out a way to take those away. Because no QB or offense is 100% invincible to having a off game or being game planed against. The 49ers are capable of making it to the Super Bowl. But also there's a reason why rookie QBs have never won the Super Bowl, even during seasons in which they are deemed to be unstoppable.