My fear is that for most apps Jasonette will be too abstract and require lower level languages like Swift/ObjC to be more expressive.

I think a good exercise would be to find a popular mobile app and recreate all of the functionality with Jasonette, developing missing features where you need them. A demo showing you recreate Tinder or iMessage by bolting a few pieces of logic together in Jasonette would go a long way in maturing the functionality and demonstrating its viability.

Have you considered weaving Parse or Firebase into this?