Your question seems to assume that all medical care is of a preexisting nature. In fact, the opposite is true and preexisting conditions make up only a fraction of the actual types of medical care a person may need. Health insurance is designed to help you get the amount and types of health care you need at a price you can afford. It really is not a matter of whether someone needs to buy health insurance so much as a question of how you can get the best coverage with premiums that do not stretch your budget.
If you tripped and broke your arm, your health insurance would pay for some or all of your medical costs. Without insurance, you would be instantly in debt for hundreds or even thousands of dollars, depending on the severity of the fracture. The broken arm was an emergency condition, not a preexisting one, and a single fracture could more than repay you for a year or more of having health insurance coverage for a single incident.
Another example would be a dependent child which was covered on your family health insurance plan. If the child picked up a case of the flu at school, her medical care would be covered, after your deductibles, copays and other fees. In most cases, the illness would not be life threatening, and could not in any way be thought of as preexisting. And that is only the child, while it could be that the entire household may need medical care, or vaccinations at the very least.
But the fact is, if you cannot be turned down for preexisting coverage, you have fewer excuses for not having health insurance, not more. Even though they are only a portion of why you need insurance, many preexisting conditions will linger for years, and may even be lifelong, which means that having health insurance will save you many thousands of dollars that would otherwise be out of pocket expenses, potentially causing financial ruin to your household.