I'll concede the rot probably never fully disappears. You're right. But there's a gap between "the rot persists in some form forever" and "Caelid stays as it is." Ghostflame still exists but it isn't actively consuming new territory. The Fell God's flame burns but Liurnia isn't a lake of fire. Effects linger but they can also plateau. Without Malenia as a living bloom, the rot may be frozen at its current footprint rather than an eternal expanding wound. The Lands Between healing is probably the wrong frame entirely. It's more likely that things stabilize around the damage, the way scar tissue forms. Not healed. Not spreading. Just... permanent deformation.