API consumers are similarly tied to the exact API contract — what you need to do to maintain backwards compatibility is introduce a different view into it (which is why I bring up /api/vX as the common pattern).

There are even richer ways to change the DB model with triggers, for instance. Really, it's only clunky because we are not used to doing it (or do not know how), but there is no technical or conceptual limitation compared to API evolution.