Simple vs complex is not a subjective matter. There are many ways of measuring complexity but it doesn't matter which we use - supernatural entities are always obviously vastly more complex than natural ones.Well, I wouldn't use the subjective word "simple";
Think of how we describe the behaviour of an agent. Something like a large computer program would be required, detailing exactly what it does in all possible situations. Now think of the "program" to describe a peice of physical matter - just a few laws of physics that could easily fit on a page.
The relative length of the "programs" for these entities is a rough indication of the relative complexity. Supernaturalism maintains that there are agents that are not merely aggregates of many simple-to-explain physical components but are primitive elements in the world whose behaviour cannot be broken down and explained in terms of anything simpler. So the world supposedly natively contains "agents" just as it contains quarks.
The naturalist hypothesis says that this is not the case and the world is inherently simple. But we can't prove this.
Yes, if we could discover a scientific explanation of them in terms of simple primitive elemets, like atoms. Then they would be debunked. But if we discovered that the phenomena were real but could not be explained in simpler terms then they could be supernatural (though this judgement will always be provisional as we could later discover a scientific explanation).Surely not "debunked". If we discovered actual ghosts, then they would be observable and explainable. Would they then become part of what we consider "natural", though?
Probably not. But we can't rule out the possibility that it actually is supernatural, i.e. that there really is no scientific explanation. It is not meaningless to believe that something is supernatural.There's stuff we don't know now. Is it okay to call it "supernatural" even if there's a possibility that it will be observable/explainable in the future?