Radiation constraints?
I know the radition problem at Saturn is much less severe than at Jupiter, but assuming some rad damage is cumulative, would this be the ultimate life-limiting factor for Cassini?
On the other hand, Titan is pretty far out from Saturn, so if it were placed in an orbit with a peri-Saturn out in that neighborhood, maybe it could last for quite a long time.
Or perhaps there would be some science value in risky close passes, like flying close to the rings again, until it eventually gets clobbered, rather than decades worth of Titan observation.
In any case, they're fully expecting to get 50 years out of the Voyagers, with RTG degradation being what ultimately kills them, so sure, why not a long-term Saturn outpost!