I've experimented with Intercooler, and it's a nice intermediate step between just enhancing a site with some jquery or vanilla JS and full-fledged SPAs.

This is almost a general theme in webdev, the middle ground is often left out, with things accumulating either on the complex or purposely minimalistic. Rails/Sinatra, Flask/Django, React/Angular... (and yes, I'm aware that it's slightly ironic that RoR is now my "monolith" end of things, given how light it was back when we were doing horrible things with Struts and J2EE)

I hadn't thought of it in those terms, but I definitely agree. I also like that I don't have to decide up front that I'm going to write my whole app using it and can add it in later on where it makes sense. I don't know what to call that, but it's nice.

I wouldn't try to write a video game or anything like that with it, but for most of what I do it gives me what I want with very little complexity.

If you're looking for a little more control but don't want to go all-in SPA, also check out Unpoly:

https://unpoly.com/