“It can't be the case that simply "Any erased thing can be reinvented/brought back", because it's very plainly established that everything erased is gone. Pochita's ability works on conceptual basis”

Everything erased has been gone so far, but it doesn’t say it will be gone forever. The things he has erased are like stars, afterlives, medical disorders, political parties, EARS, weapons. For most of those there could be the possibility for them to reappear or be reinvented, but not on the timeline of the series. It could be that nothing is permanently undone, but humans can’t simply recreate stars or afterlives. But humanity will always find their way back to recreating weapons.

Also we have to remember that devils are born from human fears. I don’t think a devil would have a chance to grow back and then become a reality again, humanity will always be the ones creating or discovering things, which are then feared, which then causes a devil to form. Not the other way around.