This could be like mortar tubes - the country with the biggest tube can more-or-less fire anyone’s mortar rounds but you can't fire theirs.

This is somewhat the case. The camera with the shortest flange distance can use lenses from any other camera given your ability to reverse engineer the protocol.

Funny you mention that...

Really aside from Nikon, most AF protocols are fairly well understood and can be adapted to other lenses.

(in the case of Nikon, sometimes they go out of their way to break others using their systems/protocols without a license...)

But you can use larger mounts on a smaller mount (with care!) May not need a bigger tube, but perhaps a longer or shorter one.