The specific savings mechanism isn't the problem - stocks, bonds, govt pyramid schemes, whatever. All this article says is that stocks aren't a magic solution like some people thought they were - the premium they had over bonds has decreased (though they are still better than SS [1]).

The fundamental problem is that any mechanism of retirement savings is a claim on the productive output of workers. When the retiree/worker ratio gets too high (which will happen), pain ensues.

[1] If SS were eliminated, stock/bond performance would become worse as SS money chases returns on stocks/bonds. It would still be better than SS today, but would be worse than stock/bond performance today.