Here's the thing. Shanahan is definitely the best coordinator Matt's ever had. I think everyone agrees on that.
Mularkey has been garbage without Ryan. Koetter has been mediocre at best without Ryan. And Shanahan had his best year with Ryan.
Now look at the other top qbs. Brees has been in the same system since Sean Payton. Brady has been in the same system his entire career. Rodgers...same system whole career.
Eli has changed systems multiple times. Roethlisberger has changed systems once. Both were followed by a bad year.