I meantioned that earlier, the bit about the drive anyways. You make an interesting point. Sin is a destroyer even when the person inside resists it (which they always have since it's been heroes sacrificing themselves to help the world, obviously they're going to resist destroying it when they become Sin). Seymour would not only not be resisting the destruction, but actively trying to do it, in which case Sin would likely become an unstoppable engine of destruction which would likely have his way with the entirety of Spira before the next Summoner even made it to Zanarkand.