Sadly no matter how many are eaten by grouper, sharks, barracuda, etc. there are videos of them living down 400 - 500 feet where there are no predators to eat them. They are like cockroaches. Adaptive and voracious.
At that depth (400-500) there are no coral reefs or the type of fish that protect and clean the reefs. Sharks and grouper live at those deeper depths too and can forage on the Lionfish there.