Immigration can’t solve all the problems arisen from low birth rates and related issues. It doesn’t stop societal aging , but just slows down its progression . Immigrants also age and will generate rights to pensions and other welfare benefits. And we can ´ t cope with the growing loneliness problem through immigrants: goods can be imported, but not relatives.
Relying all the solution of our demographic deficits on immigration would be risky in the long term, as not so many foreigners would want to come in the future to decaying Western nations, while their home countries are becoming more developed and wealthier.
Finally, a moral issue: is it acceptable to deplete of talent and some of the best people of poorer countries through immigration, fostering the separation of people from their families and local friends, because “we” Western people refuse to have enough children?
As a summary: a proper management of immigration (regulated arrival flows in quantity and educational / professional profiles and good social integration of immigrants and descendants) is strategic for the United States, Canada, Europe, Australia ……. Well managed , the host countries may benefit much from it, while helping as well to thrive many people from poorer countries: win win. Poorly managed , it can create huge trouble for natives or for all (win lose, or lose lose). But regardless of how it deals with immigration, the West needs more babies.