People denying such would need to claim that a ship going over the horizon, never to return, stops existing. When "stops existing" would introduce additional complexity.

This does seem to be the fundamental point of disagreement regarding ontological philosophy. At least, it is when the 'horizon' in question is an event horizon. It's the most obvious simple test case. When people disagree about even the most basic questions in a subject then attempts to argue about extremely complex subjects relying on the same principles is largely pointless. Yet people still try.