I'm not talking about the 200-400 1.4x, the original post was about an external teleconverter that you could attach onto any lens that swings in and out, like the 200-400 but instead of built-in, as an accessory to any EF or RF lens. That concept does not work for the reason I described, when the teleconverter is not in place, the accessory would become an extension tube as regular lenses are not designed for the long backfocus needed to swing the teleconverter out.
As you can see on this patent, the backfocus distance of a lens changes drastically when it has built-in teleconverters:
https://www.canonrumors.com/patent-dual-internal-teleconverter-telephoto-lens/
Backfocus on the lens without either internal teleconverter:
Backfocus with teleconverter 1 in place:
Backfocus with teleconverter 2 in optical path: