Well, we must distinguish between QB’s that can scramble effectively (Aaron Rodgers) and QB’s that are true runners (Lamar Jackson). The former type you keep as long as you can. The latter are almost disposable types.
Another factor is that NFL teams must choose from the types of QB’s that colleges are now churning out. IOW, they must at least somewhat adapt their schemes to the talent (and limitations) of the incoming rookies.
Evolution and change is constant in today’s NFL. I suspect that that accounts for the trend toward youth in HC positions.
