I am not sure the backslash would be big if Gmail said that a year from now you would have to pay $9.99 per month to use your Gmail ($12.99 ad-free). I mean people would complain, but would that actually give a backslash? Especially if they made it easy for people to move their account elsewhere? People are used to paying a lot more for things outside of tech.

I suspect what is really holding them back is the loss of data, and the loss of the assumption that ~everyone has a Google account that they are logged into, which means they can be traced around the web. Google also benefits from this, since its anti-bot tool will be more accurate and less fustrating to users.