If more people in Japan don't get married, there will be fewer babies born there.
As long as the percentage of people getting married in Japan continues to decline, the birth rate will decline as well.
この場合、次のように言うのが自然で良いかと思います。
ーIf more people in Japan don't get married, there will be fewer babies born there.
「もっと多くの人が結婚しないと、赤ちゃんの数が減るだろう」
ーAs long as the percentage of people getting married in Japan continues to decline, the birth rate will decline as well.
「日本で結婚率が下がり続ける限り、出生率も下がり続ける」
as long as で「〜する限り」
ご参考まで！
カナダ 
