Why would an insurer who denied all claims have any customers? I think you're missing the market competition aspect of the insurance business.

Most people do not get to choose their own insurance company, they just get whatever their job picks. Most big companies have directors and officers that personally benefit when a particular insurance company does well. The market is completely broken.

Insurance is an arbitrage leach at best, and extortion at worst. It provides no value to society.

Without insurance Market prices would drive down the cost of healthcare.