When Sin is off being dead as a door nail, what the trout is going on in Spira?

Given the fact that Final Fantasy X gives us three summoners, you would think that people would be prepared for Sin. Get all your summoners, make them get all the Aeons, and first one to Zanarkand gets Sin this time. Then, in ten-ish years when Sin rocks up again, you can take him out before he murders entire villages worth of people. I mean seriously, come on.