> The post mentions "The open source Deno runtime shows how clean and productive a modern, batteries-included, programming environment can be" so it sounds like they expect you'll be running database backed apps and not only static sites.

It suggests you'll be running a datastore, not necessarily an SQL database (the most overrated technology in existence IMO, especially for web apps where essentially none of its strong points are relevant). Storing old data as-is and migrating on read is definitely doable, and you can keep backwards compatibility to day 1 that way relatively easily. I worked on a system much like the one from "An oral history of Bank Python" that did exactly that, and had been doing so on a large scale for around a decade. Having a better-integrated datastore that can present multiple views of the same data is another way to achieve that, if you want to keep the migration out of the "application" code.