I don't see this problem as an economic issue so much as a cultural issue. Most modern families raising children need both parents to work for financial reasons, meaning they need to outsource childcare and care for elderly family members. Family units are broken into smaller and smaller pieces.
Now, I am NOT suggesting women return to the burden of caring for others 24/7. However, the current family/community model isn't working, and we need to find a better one that does not involve sacrificing women. Movements are looking at reduced working hours (for the same pay rate), freeing people to spend more time with family, pursuing new skills and learning, and reducing stress and work overload. This must be explored thoroughly and not ignored or abandoned because somebody might profit a bit less.
We do not need a more significant population worldwide. It has been estimated that every human on earth could live in an advanced, functional society with less than half a million people worldwide. The argument that we have to have more children to care for the elderly is problematic because it then becomes cyclical. Who cares for those children when they are elderly? At what point do we accept that this is not a workable long-term solution? We need better ways to support our elderly population.
We must ensure that people do not succumb to aging as rapidly as they do now. This involves better preventative health care and pathways between work modes for those who have worked in physically intensive jobs when they were younger and into less intensive roles as they get older. If people's bodies are broken, they will not age well. But this also means educating people to self-manage their health and aging more effectively.
We should also value older community members and integrate them more effectively into our communities as they age. Give them relevant roles where they can utilise their lived experiences and acquired skills and understand they still have something valuable to share.
We need to scream blue murder over our education systems, which are not fit for purpose—unless the purpose is to produce drones. A genuinely productive society would make education freely available and capable of delivering people with 21st-century skills - for all people at all stages of life. As society advances in knowledge, we must ensure everyone advances with it. We already have the capabilities to ensure no one is left behind, from infancy onwards, but they are limited to a few (why do you think people want to send their children to private schools?)
Equity and equality worldwide would end the entire migration issue. People would not move to the UK (for example) to earn poor wages and endure poor working and living conditions if everyone worldwide had similar living standards, education, healthcare, wages, etc. People might then choose to move to the UK for a few years to develop more skills and knowledge in a specific area, enable their children to experience a different culture, or even for a change of climate (for example), but equally, British people might be moving to other people's countries to do the same.