My only concern with this approach from an ecosystem standpoint is that runtimes (and more importantly, their standard libraries) can be expensive. JS is still the heavyweight engine that everyone running a browser already has installed; a world where the browser has to download a novel runtime per website is going to be hard on the end-user on the back of a low-bandwidth connection.

... but it doesn't have to be that way. Proper tree-shaking of libraries and smart caching of common resources should make it possible for that cost to get minimized or amortized.