Sure, but it’s not because of the lack of ads, but rather because they know virtually nobody uses it. I was imagining paid Gmail, but Internet “with Gmail(tm)” being a bundle they would advertise, like when DirecTV offered TiVo. Or when you can buy HBO through Amazon Prime.
But nobody uses it because it isn't as good as gmail. Before Gmail, it was a lot more common to use your ISP's email. And given the current monopoly power of ISPs I'm pretty doubtful that any such bundling would be in users' best interests.