Unfortunately for most future retirees, Bernstein is fundamentally right about the "crunch" that will come when the ratio of retirees to workers gets too great. I said the same thing in previous posts -- not as a criticism of individuals who choose to retire early, but as a warning that they had better do so with the assumption that their asset returns won't be anything like they have been historically. This further implies a willingness to work at least part time to supplement income.
I don't think that Bernstein or anyone who understands market economics is criticizing people who "save" during their working years by investing. If the investments are in "productive" enterprises, they indeed increase national productivity. That supports today's population -- including retirees -- and some of that wealth carries forward for use in the future. Unfortunately, it doesn't carry forward to the future to the extent that you imagine. "Depreciation" is not just a way of reducing taxes, but a real economic force that destroys wealth a lot faster than people realize when they are in their working years.
For example, you may build a house and continue to live in it when you are retired, but the food you eat, the electricity you use, the future dishwasher that you buy when the current one wears out, all have to be produced by future workers and transfrerred to you in exchange for the financial assets that you have accumulated during your working years. But those financial assets (including your social security credits) will lose real purchasing power if there are too many assets "chasing" to few real goods being produced in the future. Economics isn't called the "dismal science" for nothing!
Not everything is bleak, however. Certainly one of the best fundamental "investments" that we can and do make is in the education of our children, since that gives them the "tools" to support themselves and us in the future. And one major social change that is lessening the problem of supporting retirees is the greater participation of women in the workforce.