Yes i agree..
but Adonalsium's Investiture permeated all the worlds in the cosmere. On his shattering, each such investiture automatically got connected to one or the other shards based on closeness to intent may be...
Later, Shards settled on planets based on pure choice or based on these associations. However, if what you are saying is correct:
So, by this reasoning, we can also reason that all the non-shard worlds have residue investiture that earlier belonged to Adonalsium, which has not been actively taken over by some shard. So, in that way, symbolically speaking, Adonalsium still reigns on all the non-shard worlds too.
I actually do not mind this idea.. It sort of proves that if Adonalsium were a true God then he could not be killed, so, in a way, may be, Adonalsium the power was not really killed, it is pure energy and energy can only change forms but can not be destroyed..
So, the original 16 shards may not have been as successful as think they are..