If there continues to be an increase in the number of vegetarians, a lot of cattle farmers will go out of business.
If there are more and more vegans in our world, many dairy farmers will lose their livelihood.
この場合、次のような言い方ができますよ。
ーIf there continues to be an increase in the number of vegetarians, a lot of cattle farmers will go out of business.
「ベジタリアンの数が増え続けると、多くの畜産農家は廃業になるだろう」
cattle farmers で「畜産農家」
to go out of business で「廃業する・倒産する」
ーIf there are more and more vegans in our world, many dairy farmers will lose their livelihood.
「世界でビーガンがどんどん増えると、多くの酪農家が生計を失うだろう」
dairy farmers で「酪農家」
to lose one's livelihood「生計を失う」
ご参考まで！
カナダ 
