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.
Would Mozilla be able to risk being the browser that can't access Google services? I am not sure that's a realistic scenario for any of the major browser vendors. But arguably if Google's Root CA were never to gain multi-browser support in the first place, they'd never be able to launch their service to begin with.
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.
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.