I think it's about the right time for us to stop getting all the Scions en masse, but I don't think the angle is 'whole new recurring cast of characters, it's time for Scions 2: This Time There's An Au Ra'; that just creates the same problem all over again, except you've thrown out any audience attachment that exists for the pre-existing cast, and there's now even more characters fighting over the spotlight that already can't fit all the Scions.
I do think it's time to be done with 'the Scions as a whole are around for every expansion', though, because we've reached a point where there just isn't enough for them to do; in Dawntrail Estinien actively did nothing, the twins didn't really contribute, and G'raha managed to deprive Y'shtola of any ability to provide interesting hooks or character development, while simultaneously doing nothing for the actual story himself. It would've worked a lot better to not invite G'raha, Estinien and one of the twins, and keep only the people who would actually have things to do. Ideally, any future expansion should do that, and only invite the Scions that would actively contribute. If we're going to Meracydia, it shouldn't be a whole gang thing: it should be Estinien first and foremost for the dragon connection, maybe Urianger if we're using the Warring Triad to do some mysticism stuff, and maybe it's for the best if that's all and everyone else takes the expansion off.
Also, as for the actual subject matter: if we're not going to Meracydia in 8.0, I riot. There is one continent left to visit, I'm tired of waiting for it to be Fantasy Australia's turn! I don't want to sit through Shadowbringers 2 while I sit crossing my fingers that next time is gonna be the part I'm excited about. (Especially because I don't think Shadowbringers is nearly as good as people think and an attempt to repeat it will just show its flaws more than recapture its magic, but that's a separate discussion.)