From a story perspective, the first Heavensward SMN quest establishes that we can't summon Ramuh-egi to begin with, but I can't remember the details of why. From a gameplay perspective, the game works around having specific rotations and I feel like having 9 different summons/stances including trance, demi-bahamut, and demi-grandpa would be a bit too much for the devs to balance and for the players to handle.

To me, the SMN rework is a long time coming. Yoshi-P has already stated his distaste for pet classes and we've seen them dialed back every expansion. On top of that, the class has been an awkward combination of a DoT mage and a weaker version of summoner when compared to summoners of past FF games. I'm not sure what exactly I was expecting ahead of the video, but I'm happy to see big summons with a big attack upon summon make an appearance and I'm interested to see how the new stances work.