Agreed about the overriding concern of sovereign invasion, and the motivations for a potential invader are myriad: claimed "humanitarian" intervention, desire to repatriate lost citizens, trade disputes. The fact a seasteading vessel will have to trade with conventional nation states exposes it to the risk of invasion, since eventually one nation state may decide that benefits to invasions are not outweighed by its counter-arguments (international UN-style law, ethics, etc).

Perhaps a comparison may be drawn to China's ongoing desire to repatriate Taiwan.