There is still one important scenario that benefits from supporting a fallback to "classic" HTML for web sites (and apps): Bandwidth constrained environments. GMail's plain HTML version is an existing example (a link to it shows up if the JS version takes too long to load).

Anyway, for public facing sites and apps, you may already be doing most of the necessary work for SEO purposes. Letting humans access the version that you're showing to search engine spiders shouldn't be a huge burden.