Broadly speaking: if you experience dire medical trauma — say you have a heart attack or get in a bad car accident — you need emergency care or you’ll probably die. If you have cancer, you need to treat it or you’ll die. If a vital organ stops working, you need to find a functional replacement or you’ll die.

You may have multiple options for treatment, or you may not. But for many health conditions, the choice is ultimately binary: treat your ailment or die. That sounds to me like a service that cannot be substituted.