It's defensible because it's so vague.
But if it's fleshed out so that the afterlife of the damned is so unpleasant that they would end their existence if they could, it comes to much the same thing.
If it simply means annihilation, then fine. As I've noted, that's not how Jesus put it (according to the gospel authors, anyway). But OK.
It certainly is a much more defensible position.