htmx isn't supposed to replace templating engines, it's supposed to provide the "last mile" to make your SSR pages (usually rendered with some template engine) more dynamic.

I think it's a good idea. I have been writing vanilla JS for SSR'd pages for a while and inevitably reimplement some common patterns over and over (such as "load another page, select part of it, replace part of current page"), of which htmx looks to do a good job of capturing.