The problem with "just pay for data" is that mobile voice calls use such a tiny amount of data that either the cost of voice would have to drop by several orders of magnitude, or the price of data would have to be so astronomical as to make web browsing unaffordable.
This is the problem that telcos have been struggling with in terms of mobile data for years - they don't want to give up their fat margins on voice, but the actual bytes used by voice calls is trivial.
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).
