Did you not notice when they declared Dolurrh was eberron's shadowfell, khyber was the elemental chaos, Siberyis the astral sea, added the plane of baator complete with asmodius & all of the nine hells lore in order to fit the massively conflicting tiefling/admodious metaplot as is?And yet, here we are. 4e didn't try to force Eberron into some universal cosmology, but 5e is.
Thanks for clearing that up. Despite the similarity actual underground elves (something khyber lacks) another setting not just saying "X is this setting's underdark equivalent" is all the more reason khyber should not keep being called eberon's underdark.Mystara's underground has numerous names largely because its not continuous. The most popular is the world below, but also has the broken lands and the shadow elf lands. Mechanically it was much more dangerous than the underdark because it was geologically unstable and large pockets could shift or fill with poison gas without notice. Like The shadow elves vs drow it was similar in concept but radically different past appearance in terms of actual function.
The golden Corral is a restaurant chain that does a good job of meeting the needs & expectations of its target market within the restaurant industry. With that said it will never be an upscale steakhouse, Ramen Shop, gastropub, Pastry shop, fast food joint, Ice cream stand, or many other niches within the restaurant industry Adding a Golden Corral to those other types of restaurants will not in any way improve them even if they sometimes have a salad bar or something. Mortons, Legal Seafood, Baskin Robins, & many others will cease to be what they are if they suddenly shared a kitchen & dining room with the Golden Corral Buffet.... Yes there are similarities, but the differences create incompatibilities that can not be reconciled without losin what one or the other is & anything gained is massively outweighed by the losses.The D&D multiverse isn't a kitchen sink though. It's more like a Golden Corral Buffet.
You can just stick to the section that has fried foods. Or you can go to the section that has salads.
But if you want... you can have salad AND fried foods... and pizza, and ice cream, and prime rib.
Just like with Eberron, the inclusion of any other D&D world onto the meta multiverse in literally no way impacts those individual settings... unless you want it to.