This game wasn't lost by Purdy. In overtime Shanahan became one dimensional in the red zone and took the ball out of Purdy's hand. Even then the game wasn't lost. But the vaunted Niner D couldn't get one stop to seal the deal. Coaching on both sides of the Niner ball, failed and that's what lost that game. The Niners got out coached as KC adjusted in the second half and that is when the Niners lost the game. They went into half time up by 7 and didn't score in the 3rd quarter.
Purdy did his job going 23/38 for 255 with 1TD and no INTs. They had a bit over 100 yds on the ground. They got out coached at critical moments on both offense and defense. Purdy played well enough to win. Hell even the defense sacked Mahomes 3 times and got an INT as well. Again good enough to win but when it counted most the D failed to get crucial stops.
Shanahan plays passing to open the run game up. He assumes he can control the game flow with his run game. Take him out of that mode and he struggles with his calls. He has a QB in Purdy who is better than most starters. But Shanahan rarely plays him that way. This is why McCaffery is such a huge part of Shanahan's offense. Christain accounted for about 160 yds of offense (run & pass). But they still lost.
They lost because Shanahan got out coached and he made some bad decisions. This loss sits squarely on his shoulders. So what makes people think they are a shoo-in to return to the SB next year? IMO they are going to lose at least 2 of their high priced players. If they couldn't win a Lombardi this past season what makes people think they can in the next SB? This defense has been exposed. They have to blitz to get pressure against a good OL.
They beat a green Ram roster by only 7 points. This is a completely different team now. They are seasoned with an improved offensive scheme. If the defense under Shula can improve to say a top 15 unit they will be a tough team to beat. They are looking to fill holes at PK (though Maher might improve enough to keep the job), QB1 (of the future), CB, OT, WR and perhaps at edge though IMO that is TBD as they have options.
At RB, CB, LB, safety, OL (especially OT) they are drafting for depth. This roster is mostly built and now they have one year of seasoning. Time to make a SB run.
One more draft as good as last year's and this will be a deep roster that will fill future vacancies.