You do that by requiring everyone to have insurance when they're well to help cover those that are sick. That's the mandate. If you try to separate them, the system collapses under the weight of even more cost.
You weight until you have cancer to sign up? Well then you have to pay for the cancer treatment, but if you break your leg tomorrow, the insurance company has to pay.
Again previous conditions are not an issue, because they won't cost the next insurance company anything. If you get diagnosed with, say, aids and need treatment for the rest of your life, then it should be the company you were with at the time of your illness that has to paid, even if you are no longer with them.
Couple this with payment in cash for treatment, so that you can shop for the best/cheapest/whatever doctor (just as you would with car work), a mandatory disclosure of prizes and untangle health insurance from your employer and the system should be much more manageable.
Unless you're rich, you either die from lack of treatment, or declare bankruptcy and let the rest of us pay for your treatment. Neither option is particularly good for any of us.
That bankruptcy is not a magic free moment - it simply means someone else pays. Depending on the case, this might be the state through medicare/medicaid, it might be the health care provider, or it might be third party creditors.
Eventually, though, those defaults are all factored into operating costs of hospitals, the government, banks and so on - meaning you and I pay for them.
So, in the old system, we were all forced to pay for health coverage, whether we liked it or not. In the new system, we're all forced to pay for health coverage, whether we like it or not. ACA is explicit and, with extreme certainty, cheaper, since it expands preventative care on a systemic level.
But, of course, since people didn't realize they were paying for it previously, they now throw fits because the cost is made explicit.
Are you even remotely aware of what something like that would typically cost?
Most people develop many pre-existing conditions as they get older. By the time you're 70 you'll have 5 different insurance companies all treating different conditions that you developed at various times in your life.
Not to mention what happens when companies go out of business. This plan isn't workable at all.
