(I am a social democrat, not a libertarian) All models require to some extent the youth working, but not all require a part of the youth's fruits of their labour being taken and put into social security. A libertarian might say that the onus is on the boomers to save enough money to fund their own retirement so that they're not reliant on the social security safety net.

It doesn't really matter on a macro scale if you have social security doing it, or "retirement accounts" doing it - at the base there is capital and value-add (work) and you're transferring from one to the other.

Now perhaps 401ks owning stocks is effectively "lending" capital to the working-class for a fee - but you'd have to argue that.

The point is that money is still just an abstraction. When you take a step back and analyze things in terms of goods and services being the value, you end up with the same types of questions as when analyzing social security in terms of money.