By logic, rules as makes sense, and so on, that is correct. By technical RAW, it's all covered by the spell component pouch. Private demiplane of obscure ancient wizard #342? If you've got your spell component pouch, you've got the focus to go there.
I would not advocate actually playing it that way, and I'd rule in my own games that the pouch only covers standard planes, but that's RAW for you.
The magician could also just have the right focus outside of a spell component pouch, but yes, a magician without a pouch or separate focus is probably stuck in his current plane. He would presumably know the required composition and shape for each of the standard planes, though, and being completely stranded would require that ALL of them be impossible to acquire.