The problem with Y'shtola and the Other Scions is that they are too powerful. Unless we face primals, they are much better at doing it than us. So we need them to go away to get tension. Plus we have lots of characters and not all of them are equally developed. And I don't mean tits. I mean character development. So far Alphinaud, Y'shtola and Thancred are the "main characters" with Alphie being the most developed. Alisaie is catching up to her brother slowly but surely. Papalymo is dead, Yugiri and Lyse don't really have much in terms of personality. And now we don't have any Scions for Gridania. I'm also pretty sure Gaius and Alisaie won't be the only ones going to Garlemaldd. We need room for new characters.