People can eat or not eat whatever they want, but...
...the conundrum of the Western diet isn't as simple as the role of meat consumption. The transition indigenous peoples make from traditional life styles to health impacts of 'Western' dietary foods is not a simple matter of meat. Much research suggests that a major contributory factor is the density and amount of their new carbohydrate intake.
The high rates of obesity, heat disease and diabetes (and diabetes is a disease of carbohydrate metabolism) among indigenous populations seem to indicate this (and when they return to traditional foods, health indicators improve sharply).In the case of Aborigines traditional diets have a larger meat component.(But it is more than a question of meat).
As for plant based being more sustainable I'm not so sure that that is true as agriculture per se is not necessarily more sustainable than grazing or husbandry. Indeed a good portion of the planet -- including the Australian part of it -- is not conducive to the production of grains or vegetables. (Side example: the American planes were much more sustainable ecosystems feeding buffalo in their millions than growing corn. )
This issue becomes particularly sharp when you have to work out where fertilisers should come from for us to be 'sustainable' in food. Modern fertilisers are a product of munitions technology and rely on oil inputs.
Topsoil loss in Australia is consequent not just of over grazing which tends to preserve soil structure but also of the dissecation impact of agriculture on soils . Aside from the conscious loss of vegetation cover for whatever usage -- Australian soils have suffered under the impact of cloven hoofs. (Native fauna land soft on the ground and don't break up structure). Combine this with over grazing and the fickle climate with its droughts and flooding rains and it becomes a challenge to foster sustainable soils and prevent them from blowing away to New Zealand or washing out to sea.
Nonetheless, the primary problem with the Murray Darling Basin in way of water usage is the irrigation demand engineered by agricultural methods (and capitalism) and not grazing per se.
It would be more correct to say that rather than not eating meat, planting trees and preserving soils is a much better route to sustainability.
Furthermore it is more appropriate to consider carbon farming strategies (which apply both to agriculture AND grazing ) rather than plant versus animal foods.
It seems to me that 'sustainability' and health works best by combining agriculture with husbandry/meat production with plant foods. The nutritional arguments are still being had about how much of this or that is best or necessary... but the whole question is warped by the way that food is produced under capitalism and the way it is 'value added' for consumption and profit.
But no matter which way you look at it, we control neither our environment nor out gut.
Underlying this, in my perspective, is the long term consequences of grains which have fostered 'civilisation' into being. I think this is the primary dynamic of the 'Western' diet which has generated a lot of its consequences. The impact of the switch from hunter gatherer to sedentary agriculture -- from Palaeolithic to Neolithic cultures -- are pretty clear. It wasn't a good health consequence although it made a lot of other stuff possible -- writing, the oppression of women, class divisions, the nation state... -- being but a few of its impacts. That's the tragedy which is still writ large on the lives of indigenous peoples who are dispossessed of their cultures and traditional foods today.
Furthermore, the contemporary epidemic of food intolerances and allergies are responses to plant rather than to meat consumptions. While Lactose intolerance tend to be partly genetic, our guts seem to be suffering from what we consume other than Nature's foods. This is a confusing question which is not resolved by not eating meat.
I guess my general point is that the key issue isn't so much what or how much protein you put in your mouth but how it got there.