I would say 1935 with the passage of the Social Security Act. We might last a tad longer than 70 years just because our decline has been gradual.
I would be curious to know whether the Supreme Court, as it might exist after the crash, will rule SS and any successors unConstitutional. Certainly the proven impracticality and destructiveness ought to cause them to look with greater attention at Constitutional limitations.
The proper test ought to be, "Is there a COMPELLING reason to enact Social Security?" Presuming that there is nothing stopping a state from implementing it, then the obvious answer is "no".
After the crash, I doubt the Union will still remain intact as it is right now.
That's a fair assessment, as well. Switched the government from a provider of common services (roads and bridges, defense, negotiations with other governments, etc) to a provider of common means (hurry up with my welfare check!).
My parents, and especially grandparents, remember a time where turning to the government for help was regarded as humiliating, and a very last resort. I *don't* really ever remember a time such as this, except within my immediate circle of friends and family.
Defining what caused that sea change within societal norms might be the answer. But frankly, it's just a lot of navel gazing. We should let the historians handle that for us, we have work to do! :-)