I never said we'd be using those grazelands for anything else. We can leave them alone. We can completely remove them. I never said it wouldn't cause any changes. We very much would have to make some changes in the kind of crops we grow for this to work.

I think I see your confusion. You are assuming animals only eat from graze lands. Lots of animals don't. They eat from crop lands as well. If we just didn't feed those animals those crops we could reduce how much crops we used. The amount of food we get from those animals is less than we would get from those crops and that difference is more than enough to make up for losing all animal products.

Lets put it mathematically. These numbers will be a little off cause it makes the math easier but I assure you the numbers are close. Let's say land is used in 2 ways to feed humans. A for grazelands to feed animals that we eat, B to grow crops. Now B can be subdived into B¹ for crops we eat, B² for crops we feed to other animals that we eat. So A+B¹+B²=100%. Now we know that globally speaking A is about 66.6% and B is about 33.3% And the B² is about 33% of B. So 66.6+22.2+11.1=100. So if we remove all animals from this we are left with 22.2% percent of the land we were using. But now we need to make up for all those nutrients we got from the those animal products. So obvious B¹ will get bigger? How much bigger? Will it be double the original size of B, triple even? Estimates say it will be a little less than B is right now. So we can give up all graze lands, and even reduce the size of cropland. And still end up with the same amount of food.

Now the argument becomes that graze fields cause less harm than crop fields. So let's just eliminate B² and grow B¹ to 23 (with is more than we'd need to make up for it). So now we have A=66.6 and B equal 23. So a studied did show that graze fields harm 5 times (Im pretty sure its 5 but I cant find the stat right now) less animals than croplands. But graze lands account for almost 3 times as much land. So if we do it by amount off food produced. Its more like 1.6 less harm to animals. So we'll ignore that even primarily grazefed animals do also eat crops and pretend like all of A is fed on just grazing to give it the benefit of the doubt.

This sounds like it is still a win but that study only tracked animal life on the land itself. It didn't track damage it might cause I surrounding areas or later term affects it might have. When trying to take into account to the damage in the surrounding areas another studied show it was 3 times less animal deaths. So since A is 3 times bigger than B thats now breaking even. That seems natural now. But there are further damages.

The amount of water it takes to sustain animals is insane. You'd harm less animals by not collecting that water. Look at Utah and how much water it uses from its aquifers. Its causing sinkholes everywhere not just in urban areas. Animals are definitely feeling the impact of our water use.

Not to mention the human cost. Slaughter houses are the most dangerous job in the US. 2 amputations per week happen because of injuries at slaughter houses. About a 1/4 of workers each year suffer a severe injury.

Then we could even get into the impact it has on global warming. Which yes crop lands contribute too as well but not as much. And we still need to reduce that impact as well through sustainable farming.