Not every poor person has a network of wealthy people who can pay for their cancer treatment.
Not to mention, what about the old guy that's worked his whole life, can't get insured because of his age, gets struck down with cancer and then loses his entire life earnings and any inheritance for his children to pay for the treatment?
Also, if a poor person without insurance goes into a US hospital in a severe way, has an op, is stable, and the hospital know all too well that that person can't pay their bills, what incentive do they have to treat that person? They're far better off chucking them out on the street, or avoiding any kind of "better" more expensive treatments - even if they know full well he'll be dead within days.
I think every developed country should have some kind of public/semi-public healthcare, it just needs to have competition between hospitals, and be kept separate from the government (unlike the NHS here, where there's no incentive for hospitals to remain competitive, or cut costs). I.e. you pay a flat tax which goes into a separate organisation independent of government which controls the healthcare budget. Each hospital is then set up as a private company, which earns money from that fund based on patient satisfaction, number of people they treat, and so forth. That covers a standard plan, which gets you the normal health stuff and approved drugs and what not. Private hospitals then have the additional opportunity to charge extra for treatments not included in the standard insurance - so things like life extending treatments, cosmetic surgery, and so forth, and private treatments are restricted to being a certain percentage of their overall treatment.
At least that seems a good concept in theory to me, anyway - whereby everyone gets a basic level of care & hospitals have an incentive to remain competitive.