It's definitely a step in the right direction. But wouldn't the next step be vanilla JS and any-script-managed HTML?

Thanks, what do you mean with "any-script-managed HTML"? If you mean that you can use any script, like e.g. a bash script, to generate static HTML files, then yes, in a way Mastro is basically that script. Except that it comes with a server as well – both for local development and production, should a static site no longer suffice.