The problem with immigration as a solution is that unless you are going to ‘incentivise’ ( kidnapp) young children, immigrants into established Western economies are not going to be of great economic use. Leaving aside the relatively few highly educated professionals , most people who want or need to leave their countries of origin will not speak the language of the country of destination. Many are illiterate in their own language, many who are literate are using a different script. This is without considering the very different cultural differences, often reinforced by religious conviction. So the society which results will be very different from the previous one. Not worse, or better, just different.
Before I am howled down as ‘racist’,I would like to explain why I believe this. I spent fifteen years living in France, we took early retirement ( no pensions) and bought a house and imported our money. It became clear that we had to speak French, or we could literally not manage to get the services connected! We both had o level French, and were educated to degree standard. I had fairly advanced ancient languages, so was used to different sentence construction and grammar.
It was very difficult. We achieved it by paying for private tuition, every week for three hours, and practising , speaking even at our stumbling level with as many people as we could. Most of the people we knew in a similar position (including Dutch, German and Belgian , not just insular Brits) found it equally difficult, but just gave up and retreated into little communities of their own ‘langue maternelle ‘.
I suppose earlier immigration into the UK faced less of this problem, because the Commonwealth ( Empire) had offered a common language and culture.