That's not so much a concern within the kernel boundary, which is the case that applies here. If you have a compelling reason to redesign an internal API, you "just" have to fix up all the code across the tree that consumes it. Changes are regularly made to the internal VFS interfaces, for example.
It's also often the case that kernel-driver interfaces are extended without breaking compatibility. In those cases, you want to ensure that the extensions are suitable for more than one driver to consume.
