Sin wasn't killed every ten years... there must've been whole centuries where no summoner successfully opposed it. In the temples, you can see the statues of Lord Ohalland, Lady Yocun, Lord Gandof and Lord Braska, with giant statues of Lady Yunalesca and Lord Zaon rising above them, close to the ceiling.

Those statues probably represent all the times Sin was defeated. Quite sad to think that so many must've given up or been killed on their pilgrimage... the whole process is basically one of waste.