Hypermedia is what to web apps what XML is to programming languages. We have tried HTMX as a concept many times over, there is nothing new here, and like everything declarative, sooner or later it will fall short and you're going to reach for escape hatches and what not.
And the features specified in that project is nice to have, in the same way that it is nice that we have Date Pickers or other advanced input features, but it is never going to replace React-like frameworks.
Again, the reason we have finally stabilised on JSX is because you can't really "Declare" away HTML or sophisticated data and event management, Google really really tried that with Angular 1.0, and we know it doesn't scale.