One of the prevailing theories about the extinction of Megalodons (60 foot great whites essentially) is that Orca's out competed them for their primary food sources. So thanks Orcas. Without you we'd be living in a world with 60 foot sharks and that would be absolutely bloody terrifying.
Implausible that it would have changed much. People went hunting for sperm whales in small wooden boats. Sperm whales are even bigger than Megalodon. Sure they're peaceful ordinarily, but when you start throwing harpoons into them, not so much.
