Meat should not be considered a sustainable food source.
In the modern era many people have forgone animal products from their diets whether for religion, health reasons, or health benefits. While humans are omnivorous, the main benefits of meat are protein and iron, both of which now have substitute food options with the same benefits. Foods like tofu, lentils and spinach, and with todays modern convenience there hardly seems like there will be a shortage of them. Meat was a precious commodity in the past due to humans being hunter gatherers. It kept through winter and was important to survival. With the invention of grocery stores and freeze dried food there’s no longer a need for meat to sustain people. In many places there's an abundance of food at their disposal and as countries continue to make progress so too does their access to modern conveniences.  The meat industry has since been industrialized for the maximum yield for people to make money and have cheap meat but cheap does not always mean humane or safe. Cows are grazing animals but have been fed cheap corn that not only fattens them up but increases the methane gas they release, contributing to global warming.   It's not just cows though that are effecting the planet. Land is required not only for the factories but for the animals, and they're not always allowed to roam freely. Cages, closed barn doors, and a lifetime of captivity is no life for the animals people consume. The entire system of raising animals was made for speed and money, but at some point the systems going to burn out.
While vegetarianism and veganism are both suitable diets many people who are willing to swear off dog meat might not be so willing to swear off their favorite breakfast food or burger. This is due to meat being so hard to obtain in the past, giving it a high importance. As the world progressed, so did peoples abilities to raise animals. With the industrialization of the raising and slaughtering of animals, people then had access to cheap meat. Lowered prices allowed for even poorer classes to serve meat to their families. This access and excess turned meat into a standard stable at the dinner table for people spanning cultures and countries. Ease and accessibility has been the charm of today's food options from fast food to microwavable meals, many promoting tasty, juicy meat. While there are options for people to turn too, the real question is not why should they change their diets but why would they.
[P1] Meat is no longer the only source of protein for people to consume and can be removed from peoples diets.
Rejecting the premises
[PR1] Meat has been and is seen as a luxury food item.