"Better than insurance"
I don't think the purpose of insurance and the purpose of healthy living habits are congruent. Even if someone does everything you said, there is still a chance of getting cancer or being in a debilitating accident and the purpose of supplemental insurance is to assist with the financial complications of that. If someone wants to say "hey, you're better off investing that money and getting to the point where you have a sufficient safety net to cover a catastrophe," well, OK, that's a viable discussion. Both things are dealing with the financial ramifications of something bad happening.
The idea of "eat right and exercise and you won't get cancer" is not really a serious suggestion. Its sort of like saying "driving safely is better than having collision insurance." Yeah, people should drive safely, but it's not an either/or.