Do you know how far the Kuiper Belt is?
How would Cassini be able to leave Saturn's influence to get there,
and would it have enough power to do so? And where would you
send it exactly?
Why not send it to another planet while you are at it? The Kuiper
Belt will be getting attention in a few years anyway from a probe
designed to explore that region of space.
It wouldn't surprise me if NASA was trying to bump off the probe to
make room for the next crop of new space probes coming along.