Individually, you have to accept migrants if your personal demographic is below the replacement rate. Otherwise you will never retire, and your kids (if any) will work in a nursing economy at minimum wage.
It's petty and anti-nationalist to demand that your nation destroy it's own economy to fit your arbitrary political mold.
> Individually, you have to accept migrants if your personal demographic is below the replacement rate.
No, you don't have to.
This is an absurd set of assumptions that have forced you into a nonsensical position.
No, you don't need to accept migrants. You need money. You can get money in many many ways.
The best way for this to happen for the general prosperity of citizens is for the state to enrich themselves from the work of others, non citizens, and give that money to citizens. This is what Norway has done through its sovereign wealth fund.
We've now learned that this isn't true for all migrant groups, and it has been an extremely costly mistake to make. Demographic replacement can't work positively if the productivity of migrants is below of the original demographic, and actually makes the situation worse. Europe might have learned this too late, but at least now EU and most European countries are changing course.