If the Sannin are as strong as they are then other people could theoretically become as strong as them as well. They all started out as regular people (maybe not Oro...) and became legendary because they had a super badass teacher and they worked their asses off. I don't see Sai taking out Oro in any known universe, however to think no one in Konoha could ever beat him just because he's a Sanin would be the naive thing.