They are welcome to leave the dying realm. They do not, however, have any particular right to keep their regions.
A right? No, they don't have a right, their regions may still get taken away from them. They have a right, however, to try as hard as they can within the game to keep them. No one is saying Phantaria can't invade Saffalore - the game only says that if they do so, they need to fight D'Hara now.
Which may very well be what happens.
Terran is merging with a realm they are friendly with.
Terran is not merging. Terran is
disappearing. It's being wiped out. As Indirik said, there is no rule that says you must remain in a sinking ship. You take what is yours and you leave.
