What is the resistance to simplifying the playbook to get the best players playing and contributing ASAP?
And, what if Costello is hurt? Do you play a lesser talented walk-on so you don't have to play the True Frosh who has elite talent and potential?
The hope is that the walk-on actually is an adequate stand-in, but if he isn't, do you flush your season down the toilet just to preserve your - overly - complicated offense?
The goal is to win the game. Success in football comes from a combination of physical ability, execution, and scheme. The coaches have to decide how to combine these elements to win the game. If you have 10 players who can execute the whole scheme effectively, and one player with great ability who either doesn't know/can't execute the whole scheme, you can be (much) worse off than with 10 players who can execute the whole scheme and one player who can execute the whole scheme sort of OK. I think Shaw will put the most physically talented ("best" in your terms) out there not ASAP, but "as soon as it is advantageous". That is a balance among scheme, execution, and physical ability. I think Shaw does, and will continue, to simplify the scheme if it is net positive. The difference between his opinion and your opinion as to what the trade-offs are may differ. The whole thing is a big multi-variable non-linear optimization problem with many local maximums and limited experimental data obtainable. That's why Shaw gets paid the big bucks.
In your question about Costello not being able to play, what decision do you think is the right one? What if the walk-on is pretty good but not good enough to beat UW? Do you burn Mills' shirt even if you don't know for sure he can beat UW? Maybe (God forbid) the 2017 team isn't very good. How good does it have to be to make playing Mills worthwhile? And what if you do play Mills, and he gets hurt because he has no protection and doesn't know how to recognize the safety blitz? Or what if he gets so gun-shy after getting beat up for a year he is never any good (yes, this happens)? I don't pretend to know, certainly not right now, and probably not in August with any high confidence. I suspect that even during the season, there will be many "suggestions" as to what should be done.