Buying Eternity directly from trading post costs more than crafting both swords separately. Nobody would get anything for free. Actually, people keeping the other 2 swords during the process of crafting Eternity are the ones getting free weapons.
Eternity users already spent 3k+ gold for the weapon (either crafting or buying it directly). If they want to get Twilight and Sunrise, they need to spend more 3k and will be stuck with tokens for an weapon they already have crafted, while players crafting it after armory will spend only the initial 3k.