The evidence I'm proposing is that every developed country in the world has universal coverage for a fraction of what the US currently pays, which may suggest that a single-payer system is far more efficient and cost-effective.
That's not common sense for numerous reasons. I already said many earlier.
1 - you are not offering more services necessarily. All the uninsured people are still getting medical care; they just dump the costs onto you and other insured people who now have to pay higher premiums. They also dump way higher costs because they didn't have preventive care from a PCP who could have controlled their medical issues. They also go to the ED for their care which is way more expensive than if they had a PCP.
#2 - The services you are offering can be offered cheaper when they are delivered far more cost-effectively. With just one insurance company, way more money out of every dollar spent goes to actual delivery of health care rather than administrative costs.
#3 - Rationing does occur, but that's not a bad thing. Medical care that is not evidence-based, effective, or time-sensitive is rationed. For example, you aren't going to be allowed to order an MRI of someone's spine as the initial workup for lower back pain unless they have red flags. In a private system, that kind of bad medical care is encouraged because their mindset is who the F*@# cares, it means more money for the hospital, more money for the radiologist, and the patient's insurance company will just pay for it. Nobody cares about what later happens to the cost of private insurance or premiums which inevitably go way up. From an individual perspective, my health insurance costs are so much lower that if I want something done urgently I could still just pay for it privately, and still end up paying way less than if I was in the US.
These are just some reasons for why it's more effective, and explains why every other country seems to offer care for a fraction of the US. If you disagree, I am curious to hear all the reasons why it costs more money for medical care in the US. Having a larger country doesn't really make sense to me as to why that would be more expensive either. If anything that could make it cheaper due to economies of scale.