Google is not mining all networks for cleartext email. They are mining gmail which they have control of the "end". This is a huge advantage for Google as ISPs are starting to offer ability to advertise to end users by mining traffic.
So the downside to them is not that big. And it's offset by the upside of user trust and confidence being maintained, or at least eroding less.
It would also get Google a lot of street cred for being a privacy centric business.
My only problem is, I just don't see how you can scale the Chromebook software stack to a privacy centric business. I can't imagine trying to secure PGP keys on a Chromebook. But I haven't used one since the early beta models, so others may have more informed opinions,.