I'd say you can wait forever about a hypothetical FF camera that will offer no advantage to what's currently available, or purchase a camera and lenses NOW and starrt taking pictures.
A lens's focal length is independant on the sensor behind it.
the limiteds are never going to become "useless"
At worst you can sell them.
I have yet to hear one single argument that would convince me that people need full frame. If you need ludicrous resolution, then it's a medium format you need, otherwise a modern APS-C with modern lenses is much better than a non-existing full frame camera with only older lenses to work with.