Sounds to me like the obvious answer to that is to come up with a good purpose for her to move to once the subplot she originally served is sufficiently explored. In other words: character development. No sense introducing introducing a whole new character if an old one could do just as well. As for Oliver being 'the last Queen', there's no indication that this is the direction that the show intends to take. More to the point, getting rid of Thea, Moira and Walter would be getting rid of half the show's current supporting cast; which would be a major retool of the show - completely changing the character dynamics and story balance. Seeing as how the show is pretty successful at the moment, I don't foresee the show making such a drastic change in a short period of time (i.e. not in the near future).