Then i think cass would be calling the kettle black, abdicating her spot in leading the inquisition and giving it to the inquisitor in the first place
Going by the twitch demo, the Herald has already been handed the keys to the Inquisition. Well more like dumped the keys, we are talking about Cassandra here.
As for why, the demo is suggesting "figurehead" a lot, but I get the impression that Cassandra is quite fine with letting you do things your way (while she manage things has the XO and watch over you has a companion). I never had the impression she believed in luck anyhow, so she probably believe that you get the Mark because the Maker choice you to fix the problems...
Which probably result in a super awesome ending if you totally fail at leading the Inquisition in regard Cassandra's character development.