But you really can't afford to self-insure, even if you're young and healthy. There's always the small but real chance of getting a catastrophic illness and running up a million dollar bill.
Therefore, you really need to buy at least catastrophic coverage. And that's real insurance, not self-insurance.
Health insurance is complicated because for most people, it's so much more than just insurance. It's prepaying $300 a month so you don't have to pay $200 to visit the doctor once per year. No, I don't know why people do that. You're right that paying $100 a month for "please reattach my severed limbs" insurance is more affordable. Terminology be damned, that's possibly what people mean by "self-insured". They pay day to day expenses out of pocket.
Therefore, you really need to buy at least catastrophic coverage. And that's real insurance, not self-insurance.