The thing is, we aren't managing JUST for cattle. The cattle will obviously survive, but they may be making it uninhabitable for all the other plants and animals. I say "may" because I don't know what is happening there, but to be honest without oversight ranchers manage for maximum profit which will put more cattle on the land than many of the native plants and animals can handle. There is no money is saving tortoises.
If the native plants go away, then the profit goes away.
The tortoises have lived with the cattle for probably a century or more.