>or a collapse of institutions.
That's much more likely and preferable than the alternative
There is also a matter of housing to consider, big families ("big" by today standards which are flawed, so "normally sized families" by a more mentally sane worldview) require space for their kids, houses must be big enough to host a family of 6 kids or more.
But today houses, most of them, are built to only host 2 suckers and some pets, and less and less are built with a family in mind, most new houses and apartments don't have the space for the kids.
So to revert the tendency of suicidal birthrates, it would also be needed to raze 70% of the current housing lots, and to rebuild them to be bigger and with more room to host families of at least 6 kids for each house or apartment.
The immigrants are a non-problem, they all must go back to their nations, or be exterminated if they refuse to go back, no single nation in europe has "spare space" for immigrants, every space belongs to the people of the nation and of their current and future families.
Any bank or estate firm giving out living spaces to immigrants is a traitor of the nation and must be hanged, until the problem is resolved.