If I recall, they announced there would be a special story for those who completed the 7th umbral storyline. The missions ending with futures perfect were available the day 1.0 launched. New 7th umbral quests were added practically every 1.0 patch update. The 7th umbral storyline was the main story.

If you beat all of the 7th umbral storyline you received the "once in a lifetime" achievement and the Dalamud horn." "United we Stand" wasn't even the end of the 7th umbral storyline (still was a few more quests and another battlefield). Thus, technically, SE made it easier to obtain the "legacy storyline." If you didn't play the 7th umbral storyline, your character being teleported 5 years in the future wouldn't make sense; hense why you don't get the "legacy" story.