> If any process that would break compatibility was run outside Abox
> would compatibility matter?

There's no 'if' to that. It's a fact that any process that would break compatibility with ABox API would have to be run outside ABox. There's no way around that. And whether ABox API compatibility does or doesn't matter to someone is a highly subjective matter. But to answer your question eventually: Inside ABox, ABox compatibility matters. Outside ABox, ABox compatibility doesn't matter ;-)