Originally Posted by mr. peasant
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).
It would actually make the show more explosive, though Walter may stay to be the evil step dad who tries to take Ollie's company from him.
Thea and Moira cant really see their purpose beyond season 1. They may lose some supporting characters but it would only bring other characters to the forefront like Roy, Felicity, Joanna, some other characters around Ollie's age group allowing him to be more independent.