The core problem is that a not-insignificant number of mentally ill people are absolutely convinced that they are totally fine and sane, and legally you cannot force an adult into treatment.

Same with drug addicts. However an accessible and affordable system gives off ramps in moments of lucidity or desperation. Most people in moments of extreme self assurance are in an ephemeral state and that will eventually change. Untreated mental illness is rarely consistent. That's what can make it dangerous to the person experiencing it and those around them.