When I think of a real demiplane, I think of Ravenloft: The Demiplane of Dread. Anything much smaller than that is a disservice to the title.So, I looked up the spell again, and I realized it actually does kinda suck, in the way I care about, which is allowing a wizard to make a real demiplane. Here's an example of what I imagine when I think of a demiplane made by a high level wizard:
A large tower (or smallish castle) sits atop an earthburg floating in a stormy sky. If you travel more than a few hundred feet in any direction, you reappear out of the storm from the opposite side.
Level 9 spells are the most powerful magic that could ever possibly exist. If there has ever been any spellcaster within the entire history of the setting who has done anything useful or interesting, then a level 9 spell should be able to facilitate that. Because if they can't, then it creates a situation where NPCs are better than PCs, just because they're NPCs. And that's not cool.
The same could be said of rope trick, which is kind of sad, that the difference in spell levels amounts to basically no change in their functionality.As it is, this spell should really just be renamed pocket dimension.