I get the thoughts about CH and his knee issues... but Ryan is by no means a "mobile qb". Looking up his 40 times on ESPN, he was listed as 4.95 (Urich 4.84, Doty 4.62) Couldn't find Hill's, but I'm sure it isn't accurate given the above mentioned ACLs.
Ryan played hurt a bunch last year, but still had horrible footwork (Bobo mentioned that from day one). As they have said, it's not about being able to make the big play sometimes as much as it is making the routine play all the time.
I trust Bobo and his decision on this. IMHO, after this year, neither will be playing QB starting next year. Doty will take over the helm until Stockton comes in 2022.