alt Hacker NewsThis 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.
It's absolutely not an absurd set of assumptions. The majority of the demographics you're worried about are beneath the replacement rate. They are going to necessarily reach a point where the youth workforce cannot sustain the outsized elderly population or protect them in a war scenario.
If you do not want to defend Europe's front lines with teenage conscripts and geriatric riflemen, you need a way to bring warm bodies into the fold. Refusing lawful immigration will undermine domestic security, doom the elderly and ruin pension funds for future retirees. No sovereign wealth fund is worth losing your sovereignty over.