Which brings us full-circle --- what percentage of the population participating in 401-K or similar investment structures _and_ cashing out at retirement age will the economy support?

Apparently almost half of workers are not participating in such: https://pensionrights.org/resource/how-many-american-workers...

You are probably going to be OK if you are invested. If you aren't invested (no i money to invest or you save money under your mattress for poor returns), then I really don't know. It is definitely going to be a problem, maybe we will have figured out a social safety net by then (speaking as an American, and speaking in a very cynical tone).