The only thing really holding the RU theory back is ironically time itself. Ultimecia is from far in the future, and the game never once says that sorceresses have extended lifespans. In the final stretch you fight many generations of sorceresses until you get to Ulti, which would make her 100+ years old or more by the time she is persecuted and starts her kompression plan.

Rinoa already becomes a sorceress in the "present", so why would her persecution only start a hundred years later? Sorceress powers have to be passed on, they can't be shared, so where did the successive generations of witches after her come from if Rinoa still had them at the end to become the final sorceress?