Cows are a fairly major producer of methane, which is a potent greenhouse gas, contributing ~20-80x more to global warming than CO2 over the next 20-100 years.

I do think it's interesting that there are different definitions / categories of "sustainable" right now - global warming sustainable, ecologically sustainable, resource sustainable - and they do seem to get conflated together.

Are cows grown specifically for leather? If the demand for leather dropped to zero, would any fewer cows be raised?

Not to mention the large amounts of land and water used to raise cattle.