Yeah, I’ll echo Steve’s comment- Axure has had this functionality (and much more besides to deal with variables, states, and logic) for more than a decade. It’s extremely common amongst the enterprise clients I’ve worked with because it’s cross-platform (unlike that popular screen layout and prototyping tool) and usable by BA/PM/Dev as well as designers.

Sure, it’s not the slickest piece of software, but it does handle responsive layouts, transition animations, mobile prototypes, data inputs and more just fine. And it’s as high fidelity as you have time to make it.

Don’t get me wrong, I’m excited to see someone else’s take on the problem because Axure should be a lot easier to use, cheaper, and it should produce more usable/production ready assets. And the design of the dynamic features of Axure takes a while to get your head around, for sure. But once you’re there, it’s capable of easily solving the problems that you lay out.

If Phase can improve on Axure’s solution, then I look forward to using it when it’s mature enough for an enterprise environment. But don’t misrepresent the problem.