It seems that you are paying for two things with your bill, the ubiquity of the network, and the marginal amount of data you use. It seems reasonable that the capital build out be paid for with a fixed monthly bill, while the marginal amount of data usage be metered on top. They don't necessarily have to lose their margins on voice, just build that into the fixed monthly cost.
It's tricky though, because they have built out, and have to maintain, quite a large infrastructure to cope with voice alone. It extends from billing, through to terminating calls on other networks, running voice mail systems, etc.
There's arguably a lot of room for MVNOs only offering data. (In Australia DoDo offered such for the lowest price per GB).
