Universal programs are more robust than means-tested ones
Means-tested programs always engender a set of perverse incentives. When government benefits are only available to the poor or the disabled, there is an incentive to stay poor (or pretend that you’re poor, by working off the books), or to pretend that you’re disabled (witness the massive abuse of the SSI system, which has become Welfare for White People in many areas.)
Universal programs, by being available to all, avoid these pitfalls. And they are more robust politically, too. Means-tested welfare programs will always carry with them a class-based and racial stigma that make them vulnerable to political attack. Universal programs largely avoid this. Witness the relative popularity of Social Security retirement benefits, as compared to, say, Food Stamps.
This is why recent proposals to turn Social Security into a means-tested program are dangerous. As a commenter to this article put it:
“Republicans want means testing because this will make Social Security appear the same as welfare and make it easier to do away with. They can initially set the cut off high and then let inflation bring them down until only the poor receive payments. It is then easy to attack.”