Why is a declining marriage and birth rates bad?

People say it's women's fault, it's men's fault, it's economy fault, it's the end of society it will ruin everything... why? Why is lower marriage and birth rates so bad in themself?

People are still in relationship and getting married, and people are still having and raising kids, just not as often. Society won't collapse just because less people have a special ceremony or have 6 kids they can't afford.

Besides, if you really think it's a problem, why not try to fix the actual problems that most directly caused them? Things like over-working culture, climate disasters and culture of violence making people scared to date and have kids. Excessive expense of hospital and medical bills making people scared to get pregnant and give birth. Etc...

Why not fix those things instead of trying to artificially force marriage and birth rates to go up without changing any part of the environment that led to them declining in the first place? Trying to "fix" it this way will only cause or make worse real problems like unhappy or abusive marriages and unhappy or abusive households turning out criminal-prone kids.

Why is a declining marriage and birth rates bad?
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