I feel as if Rails plus a progressive framework such as Vue.js is essentially what you are talking about?

Yeah, almost. That's essentially my current preferred stack already. But "almost" is the key word: sometimes there seems to be some overlapping duplication of functionality, if not necessarily code, and the handoff between Rails-world and Vue-land is still a kind of purgatory which can be solved and navigated with experience and knowledge of the ins-ands-outs of both frameworks / webpack / package management / etc., but it'd be _nice_ if all those options were solved in the first place, rather than having to work to cleanly and sustainably get two frameworks to talk to each other nicely.