Oligopolies may seem like minting gold but live by the sword, die by the sword. Imagine a scenario where the government allows just 1 provider of some social service - like Communism. It then has the power to set any subsidized fake price it wants, usually due to populism or to appease the people to maintain power. So you get a fake or Potemkin like stock market that is really no market and you get low prices that make the investment not work out. Is a duopoly any safer? Maybe a little. Look what happened to the big banks. As soon as the government got desperate enough and wanted to get some money, it did a 'special tax' for the banks. Remember when government owns everything or near everything it becomes their go to piggy bank. To stay in power or have 'stability' they will prioritize businesses over personal income tax imho. In the end game, everyone is taxed more and more, but I think the lack of a free market is the biggest risk.