Nomads typically live in areas very poor in resources (like Touaregs in deserts) or with high seasonal variation (like Sioux in plains).

There is nothing inherently destructive with the lifestyle, and in fact many other animals are nomadic or have seasonal migrations.

Nomads also live in those areas because agrarians pushes them out of more productive land. Prior to the development of agriculture, there wasn't such a distinction.

That's sustainable if the average area per tribe that exists is much greater than the tribe actually needs in total.

That was sustainable as long as they killed each other at the same rate as they reproduced, so they didn't fill all the available area. Civilized societies and medicine destroyed any chance of sustainability until contraception was invented. Now, only the pension funds propaganda keeps us from reaching a sustainable population again.