I'm on Medicare, and it works fine.
What the guys trying to replace Obamacare with some kind of insurance situation don't seem to understand that the purpose of insurance--*all* insurance--is to spread the risk. We all pay into homeowner's insurance, even though most of us don't smoke in bed or have walls made of cardboard--if our house burns down, we are covered; we all pay into auto insurance, even though most of us have never driven drunk or had a serious accident--if we are in a wreck, we are covered; we all pay into health insurance no matter how healthy or sick we are, so that insurance coverage is there for anyone who needs it. This is why "high-risk pools" are an obscene aberration.
Insurance *exists* to spread the risk, not to generate massive amounts of money for insurers--which is why states all have Insurance Commissioners, to regulate the industry and its profits. Insurers are themselves part of the risk; if everyone files a claim at once, they're toast. They are essentially betting that the majority of the people they insure will never need their services, and mostly this is correct. When they try to siphon off the people who are more likely to use their services and charge them more (lots more), this is a perversion of what insurance is supposed to be.
Insurance companies were fine under Obamacare, what with the millions of new clients, as long as the state/federal-backed subsidies were in place to help poor people pay their premiums. When the Republics pulled that out of the equation, of course insurers quit.