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.

It absolutely does matter whether you're taxing wages or capital though.

Wages are constrained by the number of workers. Capital is constrained by total productivity.