If providing a good or service requires the company providing that good or service to have a monopoly, then that good or service needs to be socialized.

Example: infrastructure.

You’ll never have multiple power lines running down your street, letting you hook up to whichever power line is a better deal. It’s a monopoly.

Same with water, same with internet, same with railroads. They work better socialized.

If a good or service is beneficial to a country as a whole, but can’t be run profitably, it should be socialized. Examples: the postal service, disaster relief, national defense, education.

If a good or service is much more beneficial to the country when it’s free than when it’s at max profit, have a socialized option and allow companies to compete with it.

Examples: Health insurance, publishing research, funding research. These are things that will save money or improve life in the long run if they’re free or cheap.

If a good or service is considered essential to the point where the company providing that service cannot be allowed to go out of business, and the government will bail them out every time they get close to bankruptcy, that company needs to be socialized. The free market requires companies to be able to go out of business to function.

If an industry allows for lots of competition between companies, don’t socialize it. That competition will drive innovation.