If your population is declining and immigrants aren’t even learning the language, it’s not “multiculturalism”, it’s just handing the country over to another culture. Taking into account that progressive values are correlated with lower birthrates, and “regressive” ones are related to higher birthrates, are you comfortable with the consequences of this transition?
Are you sure that things like women’s rights are going to stay the same in the long term by substituting the secular population with people raised with religious values associated with high birth rates, like indians, middle easterners, africans and so on? Are you sure material conditions will remain the same by substituting the working class with immigrants from countries with poor education systems, fresh off large scale political instability?