Another way would be to enforce open-source requirements for any public spending including/producing code. You want a city to buy your traffic control system? Open source the code first. You want a state to use your voting machines? Open source your code first. Any public money going should necessitate that software being released as open source.

Not only would this lower the risk of fatally flawed public software projects, maintenance of these systems in the long run would also not depend on just one company with an artificial monopoly anymore. This would also create a healthy ecosystem of paid open source developers.

Just an idea.