The realm's not dead, nor has it effectively ceased to exist. It still has a ruler, nobles, gold, soldiers, and the ability to engage in trade, diplomacy and military action with other realms.
You are splitting hairs.
It has only
ever been possible to remove a realm's last region peacefully by bugs. Thus,
any realm merger that is not also a bug exploit must, of necessity, leave every single one of these things in place.
