With that argument the world will die soon, migration is just a temporary measure in civilization time scale. So better fix it now than push it to the next generation.
Eh, this seems dubious. If there's something structural within a nation that is leading to lower birth rates, importing excess humans from other countries doesn't actually address the root problem.
Magically, immigration is the _one_ supply shock that has zero impact on wages. Or, the _one_ demand shock that has zero impact on housing prices. The only impact it apparently does have is to make the GDP number go up.