Also, any unilateral move by Google would have to get past the rest of the CAB, which includes Apple, Microsoft and Mozilla, who all control significant root certificate stores.
Whether or not Mozilla, Microsoft, or Apple follow suit, if Google were to chose to distrust a CA, such as they did with Symantec, due to Chrome's 60%+ market share, that CA is out of business, regardless of what any other browser vendor says on the matter. Whether or not the other vendors choose to follow suit doesn't change the fact that the CAB has no legal authority over Google, and Google is a significant majority of the browser market.
Google's going to maintain a root store either way. The parent I replied to was worried that Google becoming a CA would be a power play to eliminate all other CAs and then Google would get out of the business. Apple, Microsoft and Mozilla have the power to drop Google's CA from their root stores and make Google-issued certs worthless on a huge number of devices worldwide, which is how they act as a balance against unilateral moves by Google-the-CA.
I don't think Google would "embrace, extend, extinguish" the CA market though as the parent suggested, the CA system gives Google a massive amount of control of the Internet at large, and I can't fathom them giving it up.