As the PM, you could lead a government that passed all sorts of things that benefited your family, possibly for many generations As a hereditary peer you'd get

As the PM, you could lead a government that passed all sorts of things that benefited your family, possibly for many generations As a hereditary peer you'd get to be one of a few hundred people that could vote to veto some stuff which the Commons could then override if they felt strongly enough to persist with it. And lunch expenses, I guess. I don't think the few remaining hereditary peerages will outlast this generation either...