It converts Demiplane from a purely defensive panic button/stronghold into which you retreat
Yeah... about that: the spell does not specify anything about what happens when you close the door (or indeed that the door is something that can be closed at all).
I realize you're envisioning the wizard opening up a demiplane, running into it, and closing the door behind him.
But how do you actually support that basd on what the spell actually says?
Can't the monsters just bash in the door? It has no stats, does that mean the door is a flimsy wooden door or some indestructible "door of force"? Can the door actually be closed (the spell only talks about opening the door)?
Even so, what does the spell actually accomplish except conjure up an extra dungeon room? The spell isn't concentration and isn't listed as dismissable, so there is no obvious way of severing the connection short of casting dispel magic on your own spell (while inside).
Besides, if my wizard ally casts the spell and I run inside, I sure hope no unseen monster stuns or kills the wizard before she can join me inside - doesn't killing a creature end all its ongoing spells...?
I'm having trouble envisioning practical use of the spell, except for long-term storage (as a kind of clumsy but big bag of holding). Does anyone know why the demiplane is so small by the way - it feels ridiculously ungenerous for a level 8 spell...
You could smuggle reinforcements past a check-point I guess (create a demiplane, walk in allies, close demiplane; new day, visit king, cast the spell as a kind of summoning spell that brings in exactly the allies you want without any of the usual summoning limitations).