This is reasonable advice early on, when you only have a reasonable number of warts in your API and are still able to move rapidly. However, as you go, maintai

This is reasonable advice early on, when you only have a reasonable number of warts in your API and are still able to move rapidly. However, as you go, maintaining every API in a fully-backwards-compatible-forever environment will eventually create sufficient cruft that you as the API developer can no longer build new things quickly (or, eventually, at all), which means that eventually you are unable to keep your technology and feature set growing enough to stay relevant in the marketplace because you have an increasingly enormous investment in your historical support story. At which point you eventually start losing your current customers too because they can do enough things with the new, fast-evolving API of your competitor that it's worth the cost of rebuilding.