That's not the thing here that is so great...
First of all, Final Fantasy IX was the last mainline entry that let you fly around a continuous world yourself. And the worlds only consisted of very crude, undetailed overworlds (not real worlds):
It's the immense size and detail of FFXV's world combined with the fact that it is actually freely maneuverable that is so great as well as it is a great technical achievement. To give you an idea just how huge this world is:
It is about 15 times larger than GTA5 or the Witcher 3!
We already knew of this size for some time but that somehow didn't feel that impressive because everyone thought most of the map was actually not explorable/accessable and that regions like Duscae, Altissia or Tenebrae were large but clearly seperated from each other. Probably through loading times like in The Witcher 3. The sudden reveal of free air travel shows though that this map is actually one continuous piece.
Not knowing this, many, including me, always wondered what the big deal was with Hajime Tabata repeating again and again for years how they are looking into implementing airships and that it was apparently so difficult. Months ago they announced to team up with Avalanche Studios (specializing in in open world games such as the Just Cause series and Mad Max) to implement this feature. No wonder it was so difficult, took so much time and another game studio's assistance...when they had to create the largest game world of it's kind ever.
How big of an achievement this actually is should have become clear to everyone the second Square Enix decided the reveal of air travel to be the grand finale of the Uncovered event.