in the real world there is some overlap between witness votes but actually (pre hostile takeover) there was not as much overlap between witness votes as your scenario assumes. For instance, not a single top witness had both the Freedom and Blocktrades votes, which were the two largest votes pre Stinc stake being used to vote. But as you point out, even in the very simplified and unrealistic scenario you present, being able to concentrate witness votes on just 4 witnesses to prevent a hardfork is actually a strong measure to deter hostile takeover.