I'm going to have to agree with squirtle on this one. They definitely said that their ultimate purpose was to stop Deathwing when he goes mad, and once that purpose was fulfilled, they would no longer be blessed with titan magic.
But I think what both sides seem to be arguing is the same point. Their destiny was to defeat Deathwing, that was their purpose. They were always meant to pour their essence into the Dragon Soul, so the act of channeling their powers to the weapon is in fact fulfilling their destiny set in action by Tyr thousands of years ago.
TLRD: Fulfilling their destiny = channeling their essence into the Dragon Soul. Both are adequate explanations because both are the same thing said differently.