This seems like such a critical issue that governments around the world will need to have significant societal changes - at the moment this doesn't seem like something that tech or biotech will be able to solve simply.
as you can't change the birth rate overnight, and even if you could, it would take ~20years for the kids to begin earning, there needs to be a short-term solution. and that is immigration.
good thing immigration is so popular in rich western societies at the moment.
[0] https://link.springer.com/article/10.1007/s00148-017-0636-1
What is the alternative? Just let the population age?
I would say change the culture to become self sustaining again. The best time to start was yesterday and the second best time is today.
What is the long-term one then? Keep importing people and hope for the best?
well, it normally would be, in our situation the climate likely will interfere with any long term plans.