You're the one making a lot of assumptions based on what was originally a shower thought. It's a nice thought but it's completely and utterly incorrect in reality.

My kids are real life examples demonstrating that huge investment, while good for the individuals, does not reduce the cost or burden of them to society later in their life.

This isn't an assumption, it's a data point that contradicts your hypothetical theory.

The fundamental flaw in your thinking is your assumption that treating mental illness and disabilities will result in the person becoming a productive member of society. This is occasionally true, but much more commonly, the treatment serves to alleviate the more severe symptoms of the condition, without actually curing or fixing the condition.

Healthcare is primarily about minimizing the damage and suffering caused by various physical and mental ailments. It can't magically transform people into something they're not.