Depends on how complex it is meant to be. Just like many wordpress sites that could easily have been static websites, many javascript heavy sites could have easily just have been using htmlx.

If your need really, goes beyond what htmx offers, then you may need Javascript. But in my experience people tend to use the tools they know for their job, not the tools that would be best suited.

FYI, it's easy to cache the html output of a WordPress site, resulting in essentially a static site with graphical admin, page builder, and all the other bells and whistles.