Your last point is a new argument for me and is really understated. The HTML/CSS fragmentation appears to be a historical artifact more than an architectural ideal
I'm the author of ScriptUI. I whipped it up over the last few weekends as a proof of concept. I'm looking for feedback on whether you'd use something like this. I hope it helps make starting projects easier.
There’s utility for different deployments per branch and rollbacks when you’re connected to a headless CMS (Contentful, Wordpress API) and managing more complex statically-generated web sites.
idk if GH pages has added this, but Netlify lets you determine your build command and environment for deployments. You can also have different deployments per branch with easy rollback functionality. In short, its built for this exact problem where GH pages is a tacked on feature for repos.