Earlier quoted context omitted.
So the crisis is that there are not enough products and services for an aging population. That's a different crisis than low birth rates, that should be solved differently.
Well until we have robots which can fully replace many/most jobs people do, the only way solve that is by increasing the proportion of working age people to those who are retired. There are no other options. You can do that by either increasing immigration or birth rates. That’s it.
Most people's jobs aren't necessary for a functioning society. If you look at say, agriculture, transportation, energy, sanitation, infrastructure, etc, what percent of the total population is employed in those fields, 5-10% maybe?
Society will be fine for a long time with drastically dwindling population, we'll just see a contraction of the "bullshit jobs" which have become the mainstay of post-industrial societies, with a higher percent of people employed in fields which are actually producing something of value for society.