The ECB isn't stopping with government bonds. It will end the current monetary experiment with widespread intervention in corporate debt and all aspects of the market, further destroying what freedom still exists in debt markets.
Even if a machine were technically capable of providing the same service more cheaply, many consumers are likely to prefer human staff in many cases, including nurses and care providers, entertainers, chefs, and teachers.
The Fed is prepared to squeeze out what little is left of the free market forces in the debt market. The Fed wants full control so it can do "whatever it takes" to keep interest rates from rising above its very-low targets.