Sam may have signed all the chairs that left his shop but I though he had a number of employees that actually did the building of the chairs. Anyways your right something is only worth what someone else is willing to pay. 20k still seems a lot to me for a single rocker but then again I've not had the chance to sit in a real one.
It does seem that the value does diminish after the original creator/master is gone. This seems to apply to not only wood working but other areas as well. Outside of wood working I've not seen the products come anywhere near the value of the original even it was built by the same helpers that made the original. So it would seem that to be truly authentic it needs so be signed off by the master.
David