Perhaps I should have said non-working instead of dependent population. The common definition of the dependency ratio is "an age-population ratio of those typically not in the labor force (the dependent part) and those typically in the labor force (the productive part)" and this is why I used that term.
The point is, no matter how rich they are, everyone who is not working depends on workers to produce goods and services for consumption.
For over a century developed nations have decided the dependency ratio in the private sector with things such as mandatory retirement and industrial pensions, and in the public sector with programs such as Social Security.
In my opinion it is a legitimate question for public debate as to what the most desirable dependency ratio should be. Social Security is (only) one mechanism to achieve the desired ratio, but an important one. Let us decide what ratio we want, then debate how to get there.
I gather there are some who would rather let the market decide all -- if you have enough funds set aside you can stop working, otherwise...