> a lot of the lands used for grazing simply aren't viable for much else, and thus we couldn't really plant anything else there.
Perhaps in the some places, but 80% of deforestation of the Amazon rainforest is for cattle ranching. [0]
Here in New Zealand, European settlers managed to clear 27% of all forest in just 10 years [1], replacing it with farming, which is what it's still used for (especially cattle, these days, and obviously they didn't stop clearing forests at 27%, they kept right on going). This is land described by early European explorers as "immense woods, lofty trees and the finest timber" [2]
Also, land doesn’t have to be productive for business. We could take the cows away from land that’s not viable for other farming and just let the land be. (My guess: given sufficient recovery time we might be surprised at what does grow)
[0] https://wwf.panda.org/knowledge_hub/where_we_work/amazon/ama...
[1] https://www.mfe.govt.nz/publications/environmental-reporting....
[2] https://en.wikipedia.org/wiki/Deforestation_in_New_Zealand#:...