So I was thinking about agency and party makeup, and I think one of the issues with Vivienne is that you can't get rid of her.
I started going through the history of how you can engage with your fellow companions throughout the main game series, and I came up with this list:
Companions you can kill:
Leliana, Alistair, Wynne, Zevran, Shale, Loghain, Oghren, Sten, Bethany!, Anders, Merrill, Fenris*, Blackwall*
Companions you can enslave:
Fenris*
Companions you can imprison (success not guaranteed):
Morrigan*, Isabela
Companions you can send away/drive away:
Morrigan*, Aveline, Carver, Blackwall*, Sera, Cole, Dorian
Companions you're stuck with to the bitter end no matter what:
Varric (both times), Solas, Iron Bull, Cassandra, Vivienne
Now for those you're stuck with, three are arguably plot driven (Varric in DA2, Cassandra, Solas). So of the three characters of the entire series of games you're stuck with for no reason and have no way of getting rid of, they consist of a wisecracking easygoing dwarf, a wisecracking easygoing Qunari mercenary, and a manipulative arrogant mage.
I think there's traction to the idea of how the writing and market of this game series has changed considering you used to be able to kill your sister for role playing reasons, and now you can't even kick out a character who is openly challenging you in your own court.