Sustainable here has multiple definitions
While we hope to never ship broken code and have a lot of layers of testing involved, our definition of broken can extend to accessibility issues. Because we are committed to highly semantic, readable code that traces back to unbundled tags, it makes it easier to fix things without updating them in the HAX sites themselves.
Past releases are able to update the definition of simple-cta
to improve the accessibility after the fact, without a developer, or content author, ever going to a site to make the change in the HTML.
With more people involved in the web's creation, we can more easily maintain and improve upon what's created. See Community improvement pipeline for more details.