I can't argue the success of SS on the aged but I can argue that liberal policies alone are not the reason for the drop in the poverty rates in general. I do wish you had posted a more current graph but does show what you have stated.Liberalism never claimed it could eliminate poverty. It was stated as one of our goals. But no ever said we knew for certain that we could completely eliminate it. The War on Poverty was actually quite successful.
I'm certain you want a link to this info. It would be better, though, if ya googled it yourself. My links would probably be biased in your eyes. But take my word for it, The War on Poverty was successful. Just as SS has been phenomenally successful at reducing the senior poverty rate.