"Stuck"? That could mean different things in the context of your question...
It could mean that Cassini is unable to break free of the planet's gravity to go to another planet. I suspect in that sense Cassini is already stuck because it doesn't have enough fuel to do that. Not that anyone wants it to anyway.
But stuck could also mean unable to stay up. Orbits can be low and fast or high and slow, so distance isn't the issue. It all comes down to having the fuel to maintain a stable orbit. Cassini is gradually using its fuel to, from time to time, change orbits so it can have a look at different aspects of the Jovian system. It does so very carefully, using the gravity of various moons to assist its changes to minimise fuel use. But eventually it will run out of fuel and will need to settle in somewhere for the long haul. That final orbit location might be very high so as to be stable for a long time, or they might decide to send Cassini to a dramatic and quick death so as to collect data on the way down. I suspect they haven't decided yet.