FormerlyHemlock
Hero
So... 15th level at earliest? And honestly, if that is the most powerful and most clever use of Demiplane a player can come up... meh?
I'd characterize it less as "meh" and more as "interesting" and maybe "occasionally useful." It converts Demiplane from a purely defensive panic button/stronghold into which you retreat, into a panic button which you press to retreat and then optionally re-emerge from a few seconds later covered in Haste, Foresight (maybe), Mirror Image, Stoneskin, Protection From Elements, and with a Simulacrum buddy to double your firepower.
The other interesting use of Demiplane I can think of is actually the zombiepocalypse (Demiplane stuffed full of Necromancer-charged skeletons and zombies which are now uncontrolled and omnicidal; you Teleport in, cast Demiplane to unleash the zombies, and Teleport back out).
I see how it could be a concern, looks like you could store up to 30 glyphs within a demiplane, break concentration rules and buff your entire party with a whole suite of abilities. Costs 6,000 gold a minimum though
Or store 216 mephits inside to unleash in a horde of exploding terror upon an enemy
Or literally anything else you could think of that fits in a 30x30x30 room. I honestly could see Demiplane as one of the most powerful spells in the game for how flexible it is.
Yeah, potentially it is very good in a Combat As War-style game. Even just the potential for tricking General Zod into the Demiplane and then Dispelling it to send him permanently into the Phantom Zone... it's a nice little spell. Probably overkill for a Combat As Sport-style game though, wherein reducing an enemy to zero HP is always good enough.