Love this a ton! How does Netflix ensure reliability for those platforms with an ever growing codebase and the evolving business? Was the platform built to be extensible from the start?
Most of the old apps can't be updated, so they are sort of frozen in time. They all hit APIs on the backend, so as long as the APIs they hit remain backwards compatible it "just works". The old apps don't get any new features that require UI changes or can't be supported on the platform if that platform is updateable.
So the short answer is, everything is an API, so as long as the API stays backwards compatible it just works.
