Monopolies are bad only because the winner in a monopoly nearly always uses their market position to increase the barrier to entry to upstarts, often by lobbying for increased regulations (costs) in the market that didn't exist when they started. If monopolies (at least some - Amazon works as an example) operated in the same way they did on their way up - looking for ways to disrupt and improve on the status quo rather than looking for ways to ensure the status quo does not change, they wouldn't be nearly the negative force that they generally are.
