I can see the appeal in having one ontological world. What is that world, exactly? Is it that which can be proven scientifically (in the sense of, through the scientific method used in natural science)? I think what can be proven scientifically is perhaps what we are most sure is real or true. But things that we are less certain of being real can still exist, as part of the same ontological world. The uncertainty is in us, not in the world. One simplistic definition of natural science is that it is simply rigorous empiricism. The rigor isn’t how we are metaphysically connected with things, rather it’s the empirical that does so, the experiences contacting or occurring to observers. The rigor simply helps us interpret our experiences.
We can have random experiences that don’t add up to anything. But maybe whatever experiences that give rise to our concept “morality”, which we do seem to be able to discuss with some success with other people, and have done so in different time periods, may be rooted in a natural reality (which is not part of the deliverances of “natural science” as “natural” is commonly understood, but which is part of “natural science” if by “natural” we mean “part of the one ontological world”). Morality is something we try hard to make a science of (hence the field of ethics), but which to some extent eludes us. But that doesn’t mean that there isn’t something natural there, but that it’s something we have so far not figured out.