The challenge of sustainability is based on understandings of complexity, dynamics, incomplete knowledge and contested values. In human diet, however, that challenge is relatively easy to solve. Healthy diet is also good for nature including whole ecosystems.
A responsible person or society would aim to reduce meat intake in order to
(a) secure food for everyone by minimizing grain feeding to cattle,
(b) treat domestic animals in more humane ways,
(c) combat climate change and biodiversity loss,
(d) moderate harms on land and water, and
(e) prevent serious illnesses of humans.