Well, perhaps we will get an "iMessage Voice", but just like regular iMessage the carriers *still* require a text messaging plan and in most cases an "unlimited text messaging plan that runs $20.
I don't know what you are saying. First, we already have data only messages, voice, and video/voice, all supplied by Apple and built into iOS. Second, texting via SMS does require a texting plan, but just stop using that legacy way of texting. Install something like Google Voice which will do the texting for free, even to SMS, over the data connection.
I think the assumption that just because we have voice and messaging apps that run over data connections that carriers will allow data only smartphones. I just don't think that's gonna happen soon. If carriers ever do allow it, then they will make sure your data bill will be $120 a month.
Well almost all carriers except for perhaps AT&T allow iMessage and FaceTime over their data connections with no additional charge. So it isn't a question if they will ever do it, rather the only question is when will AT&T join the club?
Also, if you are cavalier, you can also just purchase a data only plan from an iPad for example, and use it in your phone thereby going data only and avoiding all the costs for the legacy way of communicating.