It really makes me sad that because of the abysmally poor character writing in Dawntrail, people get to the conclusion that the "Scions' arcs are over", that "they have nothing else to offer to the story", and that's enough of them.
It's simply not true. They had no reason to be in Dawntrail, and were actually detrimental to the situation...because they were badly written there. There was SO MUCH potential, if you get the concepts of the situations, and had written them in there with care. They all had past experiences and thoughts and feelings to add to many of the themes of the expansion, if only the writing wasn't so amateurish. Instead, we just got cardboard cutouts that sounded all the same, talked about the same things and reacted the same way to everything, and immediately left us alone with Wuk Lamat.
In Shadowbringers, no one thought any of the Scions didn't have to be there. But most of them had already gone through previous, resolved arcs in the past and that wasn't a problem. They had new things going on, and they felt important and relevant. Even in Endwalker, even if they didn't have anything major going on, their contribution to the story and the moments they were there felt right, in character, and relevant.
If you or the current writer can't think of good ways to use those beloved characters that still have so much to offer, that doesn't mean it's impossible, that's just means we need someone else writing the story.
I agree though that there isn't a need to force ALL of them to be there all the time for no reason. I'd rather not have them there than to have them not behaving like themselves and acting off, because it's just sad to watch our favorite characters get butchered by the writing.
Also the Trust excuse doesn't cut it. In ARR, they included random NPCs for story, and sometimes they can add other important NPCs as companions during the story for dungeons when it's relevant. It's not a technical limitation.