A lot of interesting points.
I don't like the argument about money, because I take you back to Matt Flynn. Why would you pay him all that money and not use him? Well... because you have something better. It would be ridiculous to go with a guy just because you paid him if there's something better.
Don't believe in the money argument and am so thankful that Pete and John let people compete. Otherwise, there is no incentive for guys not earning top dollar to do any better if they know that they will never get on the field because someone is being paid more.
As far as the rest of the points, the biggest one I agree with is ego. What does it do to the confidence of your guy if you pull him out of the redzone? Thus in my original post I mentioned the ego of your starting QB.
The second one is rhythm. Agree here. I have even heard o-line interviews specifically about cadence. QBs have a rhythm and cadence that O-lines are used to, and it is disruptive to put a new QB in with a different cadence. That in itself is what makes it very different than the one-on-one scenarios with baseball and closers and relief pitchers.
Yup, definitely not a viable option when it comes to something so team oriented as football.
I do love the idea of possibly other specialists, not necessarily the QB but RB, playcaller, coordinator, etc.
And heck, take it one step further, since thinking about it now we seem fine from the 20, it's from the 10 it seems more apparent: goal line specialists from inside the 10.