Natural monopolies exist, for sure. Your insurance example is odd though - insurance markets are generally highly competitive. The recent cases where we’ve seen a loss of competition in the market (CA home insurance, for example) have been driven by regulators imposing price controls.
The issue with healthcare is that providers have leverage over insurers, not that there is a lack of competition for insurance.
The issue with healthcare is that providers have leverage over insurers, not that there is a lack of competition for insurance.