But that feels like it's mostly a theoretical/money-issue (at least in the industrialized nations), because as people age, their consumption also changes. I live in an area with plenty of old people who slowly die off and get replaced by young families. The old folks are low-activity, have at most one car, and even that seems mostly stationary except for the occasional grocery shopping.
At the same time, productivity has risen so sharply that it's absolutely not a problem to provide the basics (food, shelter, energy) because we're far away from 20% of the population working in agriculture. Healthcare and elderly care in general becomes an issue, but that feels like it's much more manageable, e.g. we could cut cost and not give hip replacements to 80year-olds.
So this feels less like a "omg this can't work, just look at the numbers"-issue and more like a renegotiation of the intergenerational contract. Granted, that's not an easy task either given that the elderly get a vote too, but it's no longer "how would we ever accomplish this" and instead becomes a political issue. Since many elderly are also at the wealthiest stage in their life, I feel like that's not an unsolvable riddle.
What am I missing?