I think for a lot of business cases they are going to replace js frameworks. They are domain specific (Rails, Phoenix, etc..), but they allow you to essentially replace a lot of SPA functionality with a library that takes an afternoon to grok for a back-end dev.
They won't replace React, Vue, etc... But they can replace it in places where a full js framework is overkill (which is probably a majority of the places that I see it).
As a backend dev, I think that they have incredible promise. They make interactivity MUCH easier.
