Livestock breeding isn't inherently unsustainable. E.g. you can have closed-off poultry or pork farms with greenhouse-fed animals, all powered by wind or solar, with sewage processing to ensure there's no external pollution. This isn't sci-fi, it exists today in e.g. the denser parts of western Europe.

Yes it's costlier than beef from the developing world raised on cut-down rainforest land, but growing lab is a lot more expensive.

You can argue that it's inefficient, sure, but so is a vegan eating an avacado instead of white rice. Things can be relatively inefficient and still be sustainable.