You're picturing a world where the only land that exists is forested land and you have to choose to either leave it a forest or cut it and use it for agriculture or grazing. But a lot of land is not forested, and not suitable for agriculture. But it is suitable for grazing. This is the otherwise worthless land that can be converted by cows into food.
As I said, land use issues where you replace forest with cows is a problem (though not in the US, it is a problem in the developing world).
It's not about replacing forest with cows. It's about replacing forest with cow-feed.
1% of US cows are grass-finished, the rest eat grain.
These grain-eating cows may be located in a place that has grass, couldn't be forest, and couldn't be agricultural land. And yet they eat grain nonetheless, from another place, that could be agricultural land, forest or otherwise nature that creates biodiversity.
The grain comes from somewhere. That creates land use issues. Not because of replacing forest with cows, but because 99% of cows sold in US stores are eating grain which must come from somewhere that is obviously not only useable for growing just grass.
Yes, in theory it's possible to have cows eat grass only, that's quite obvious. But if you were to limit beef in US stores to only those type of situations, you'd have to reduce the supply by 99% today and radically shift the way beef is produced worldwide.
And even that 1% / 99% isn't the entire story. That only tells you what is grass-finished vs cows that eat grain. An even smaller portion of that 1% are cows that are eating grass from a place that could solely grow grass and could not be used for other purposes such as forest.
Grass-finished. Think about what that means. Most (all besides veal?) eat grass. They finish them (fatten them before slaughter) with grain. That 99% isn't 100% grain. It consists of lots of grass.
As I said, land use issues where you replace forest with cows is a problem (though not in the US, it is a problem in the developing world).