Apple is welcome to do what they like (and it seems like they're offering alternatives that are at least equivalent, and possibly superior) but a "major release" has incompatible changes to APIs by definition.
iOS major version bumps frequently though...
Besides, Apple's major OS releases are executed like a major version number bump regardless of how others outside the company do things. These bumps include deprecation markers and warnings for APIs with planned retirement. There's no technical reason to remove deprecated APIs (keeping them around forever) to support older apps.