That's why it should be destroyed by the Sith, or at least caused by something a Sith/multiple Sith did. The problem is they made too many of their characters too op. Vitiate, immortal. Nox, immortal. Revan, never dying. Scourge, immortal. That in itself causes inconsistencies.
Vitiate doesn't die of old age. Truly immortal? Not quite. After his defeat at the Dark Temple, in fact, his own vulnerabilities are thrown into stark relief, and the Empire he so painstakingly built begins to fall apart, with Malgus and the Dread Masters rebelling, and (high hopes mode on) the true Old Sith showing their ugly faces again...
No indication that Darth Nox is immortal, even with his powers. One who knows what he is up against could succeed in killing him (say, the Barsen'thor, who knows how to deal with ghosts and their proxies, or powerful Sith that use ghosts for power too, by extension).
Revan never dying has not been proven fact yet, and (high hopes mode on again) these expectations will be debunked soon. Anyhow, the only thing that kept him alive beyond his time was Vitiate, and now Vitiate has no more interest in doing so. Ergo, his death is soon to come, even if he resurfaces.
And Scourge... well, I don't know to what extent he remains immortal after his rebellion (never played through JK story to learn more about him, so...). Besides, despite his undying status (different from true immortality, that doesn't die and can't be killed), his power isn't the greatest, and he is also committed to the defeat of Vitiate.