One of the neat things about Eberron is that the planes are directly relevant to the setting. Not via planar travel, but via "manifest zones" which are locations where planar energies leak through, which essentially works as a natural resource. These zones express themselves in different fashions. For example, one zone connected to Fernia (the plane of Fire) might cause fires to burn hotter. Another might cause tempers to flare easier, or make it easier to forge bonds of loyalty.Basically, yeah. It's great that beholders exist, in the abstract, but until they directly impact the game we're actually playing, that they exist is academic. Same with planes and cosmology. I guess the D&D philosophers have to have something to argue about, but beyond that, either it's actively, directly being used in the game right now or it's indirectly affecting the game from "off screen," or it doesn't matter.