in Middle-earth (except, it seems, for the orcs...). The spirit lives on, so all death is "disembodiment" to that extent. In fact, the Ring-wraiths are already dead (as we humans define it) - already wraiths, or spirits, with no real bodily presence. Their creepy power lies in the fact that their spirits have not left the world, but continue to haunt it. Once they lose their horses and garments, they lose their "shape", but they are still in the world, as we hear from Gandalf:
"I think that we may hope now that the Ringwraiths were scattered, and have been obliged to return as best they could to their Master in Mordor, empty and shapeless." (The Council of Elrond)
But when Merry and Eowyn finally defeat the Witch-king, even his spirit seems to be banished from the world:
"a cry went up into the shuddering air, and faded to a shrill wailing, passing with the wind, a voice bodiless and thin that died, and was swallowed up, and was never heard again in that age of this world." (The Battle of the Pelennor Fields)
That sounds like much more than the mere loss of garments that happened at the Ford of Bruinen. And it fits in with Tolkien's comments on the Witch-king's fear of the barrow-swords:
"...[the Witch-king], the great captain, was actually dismayed... above all the timid and terrified Bearer had resisted him, had dared to strike at him with an enchanted sword made by his own enemies long ago for his destruction." (
quoted by squire)
You may certainly be right that if the Ring had not been destroyed, and Sauron had regained his power, the Witch-king's spirit might have been recalled from the void. To that extent, nothing really dies - evil can always return. But it can be at least temporarily banished from the world, and I think that's what happens on the Pelennor (but not at the Ford of Bruinen).