If you had no profit motivation then there is no incentive to do the best and most efficient job. This is why government organizations are always extremely bloated, inefficient, and employees don't care about the service level. It's because a monopoly means you don't have to care about any of that anymore.
The insurance company profit motive only achieves them maximizing their profit margin at all times. Which is done by charging as much as possible and blocking services as much as possible so they don't have to pay.
