Financial head - they'll fetch good money in the fat, cull them.
Only you can decide whether there are other factors to take into account. One of my farming friends had a flock of Lleyns and hardly ever had any prolapse. Then one year, nearly a third of them prolapsed. She kept them all on, and none of them prolapsed again. But to counter that, one hears more stories about keeping the ewe(s) that prolapsed on, and wishing heartily that one hadn't...