I'd rather have the often loaded static html running on a server that is optimised for that job, or served from a cache close to the user. The backend can then just serve up the dynamic content and be optimised for that job.

The 'often loaded' static html won't help if the 'often loaded' JS bundle is re-deployed (eg, when the devs make a 'small' change) and the cache is invalidated. In that case all the users will be forced to reload the giant JS bundle over and over again, as opposed to just reloading a single resource on a single page like they would with the htmx approach.