There's not enough immigrants that actually are of financial benefit to Denmark. Only European immigrants are a large financial benefit throughout their lifespan, and Middle Eastern immigrants are a financial detriment even in their prime working years[0]. That is apart from the cultural effects of having a large portion of your population consist of non-Danish, and especially non-European immigrants, which might have its own detrimental effect on the birth rate.
[0] Figure 2.7 of the Danish Government report "Økonomisk Analyse: Indvandreres nettobidrag til de offentlige finanser i 2018" - https://fm.dk/udgivelser/2021/oktober/oekonomisk-analyse-ind...