For me, with the expectation that the prices of products and services would continue to rise as well as the value of bitcoin, for a person retiring, who would no longer invest, 15BTC would be enough to leave a comfortable life for about 30years

For someone that wants to leave legacies for the family, 25BTC would go a long way maintaining the comfort of the family for a while after your demise.
That is, about 10BTC would go into investment to yield great fortune for the livelihood of the family