And then Caldwell never went on to do anything with any other QB, save for an end of season stretch with Flacco that got Flacco paid. Interesting enough, they fired Cam Cameron and let Caldwell take over that season. I mention that because Cameron's only success came with Brees and Rivers.
I think it's more about the narrative that these coaches can just take anybody at any time and put up the same numbers--the QB matters. Purdy has made a lot of people forget how Shanahan did with Hoyer, Beathard, and Mullens (and I suppose Lance). Shanahan is 62-49 overall in the regular season, but it you break it out by QB, he is 9-29 without Purdy/Jimmy G and 53-20 with those QB's. It's a huge difference in performance based on who is taking snaps.