I think I am saying the same thing as the others here, but I want to avoid pointless distinctions and nuance.
Put less sarcastically, such a kingdom has existed in the minds of many literally-minded Fundamentalist Christians now and in the past. It is part of the description of heaven, which is miraculous rather than magical, but beyond the sky and not directly accessible to us until we die. Try telling them that it 'might as well not exist'. Likewise, if your Vedanta Hindu friend tells you that the computer you are reading this from does not really exist, he is likely to be serious in some sense.
I would suggest that it is pointless to pursue any given definition of existence. Any statement of existence is a belief. It is, therefore, modal: fictional, artificial or theoretical. It is tied to some rule, desire or theory under which it is true, and outside of that context or an adequately similar one, it is not true.
The theory that what you see exists, is, in fact, a theory. It is one that all but the most bizarre skeptics, the most refined religions, and the most abstruse sciences adopt, but it cannot be proven true. It can only be proven useful, and 'useful' depends upon your context.
To frame it from a Kantian (or modulo vocabulary, Vedanta) point of view, the only non-modal existence that would not be theoretical or fictional at some level is "noumenal" reality. But that is not accessible to us except by its effects upon us, which are not the actual things doing the affecting, and do not necessarily represent it authentically. So we cannot reliably say anything meaningful about it, setting aside mystical revelation and the impossibly perfect guess. So in a practical sense, noumenal reality itself might just as well not exist. Reducing us to exactly zero things that don't fall into the category of what "might as well not exist" in one sense or another.
Being incapable of interaction removes most uses. So theories that presuppose the existence of things with that property have little point other than inspiration or logical cohesion. But inspiration and the need for logical completeness are powerful forces. So they are pretty common. The three examples above (heaven, non-maya and noumena) have many parallels.