This isn't new. Play calling ability was already taken away from him last year.
Am I wrong on this?
Garrett called the plays last season and has for the last 6 years.
What some people are forgetting is that when Callahan was hired before last season as OC as well as OL coach, there were comments from Garrett to the effect that they would give Callahan a year to learn the system and then give him more responsibility including calling plays. It had been recognized that Garrett was trying to do too much on game day and that change would/should help with game management issues. Well, here we are a year later, and he's getting additional responsibility.
Now, you can argue that the manner in which it was announced was ridiculous. I'm with you there. But such as it is.
But what about impact? Jason Witten on twitter today or yesterday said paraphrasing, "its the exact same offense, just a different play caller. No big deal at all." So in one sense, the impact is minimal as Romo ultimately chooses whether to audible out or not, something he does regularly.
Last year was Dallas' most imbalanced in its history as far as run/pass balance was concerned. They have not been able to run effectively in short yardage or otherwise in several years. They were behind in a majority of their games early and often last year which further skewed the run/pass mix.
To me, the bigger impact will be the personnel changes, especially on the OL. With the drafting of Frederick, Dallas now has a center that won't get ragdolled on a consistent basis as has happened since Gurode moved on. There is (hopefully) improved health of vet guards and development of younger OL. Tyron Smith has some major off field distractions behind him now. There is a very real chance that the OL will be materially improved. If that is the case, then Dallas should have better run/pass balance and may be able to score more by run. Other personnel changes may also impact things. Miles Austin may have finally gotten his hamstring problems resolved. Dallas used 2nd and 3rd round picks on TE and WR plus development of existing WR3 and TE2. Not to mention Dez's continued development. These could all lead to a more diversified and higher scoring offense.
Bottom line: Issues other than the play caller will have a bigger impact on Dallas next year.