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.
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.