Why would most js heavy companies want content to be long lived? What's the benefit to them? If they're concerned about making data accessible then they'll open an API that they hold the keys to.
Obviously if a company wants tight control over their content then this may work to their benefit - although they may still find that their ability to render their own content requires non-trivial maintenance over time.
But that's a pretty narrow view of the web, and not everyone publishing on the web fits that model.