@inlinable is still its own meaningful thing. The equivalent (in library evolution mode) would have been @export(interface, implementation), but it was decided not to support that in the @export attribute.
That being said, I think we should be really cautious about adding yet another knob here for things like @inlinable or @export—we certainly don't want to complicate the story around those attributes further. How would something like @export(value) differ in practice from @export(implementation)? For a simple value property, the value is the implementation.