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.
There are no other options. You can do that by either increasing immigration or birth rates. That’s it.
It’s essentially UBI and universal healthcare that winds up making a lot of demand for younger workers. Keeps the grandparents able to help raise kids, if wanted/suitable.
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.
Second, this "crisis" is entirely brought upon themselves by the generation that will have want for people to "support them". Cut out elderly healthcare and the majority of your shortage is gone.
Third, there are plenty of people available for this support, as many of the jobs these youngsters do today are only done because they are very attractive in the current economic system. If the population as a whole agrees that this "crisis" is indeed a crisis, there is enough labor to step up.