The Ultimania guide states that sorceresses have normal human life-spans. Rinoa cannot live into the time of Ultimecia and 'become' her. As much as I think that would be a cool plot twist, there's no mention of it in the game, and the Ultimania guide basically debunks any theories on it.

Yes, I'm well aware that someone may quote me with whatever insane theory they have, but it isn't my fault people can't leave things as they are.