If everyone were vegan, there would be apparent changes. Let’s see how they pan out. Suddenly there would be vast tracts of farmland, at the moment dedicated to raising meat animals, that would no longer be needed in the same way. They would either be abandoned, and revert to scrub (which eventually becomes far nicer-looking forest), or they would be turned over to crops.
Interestingly enough, a lot of the world’s current problems would diminish. The Western meat and dairy diet are impacting the world in terrible, devastating ways. Acres of rain forests are mowed down every single day to make room for more animal farming. In 2004, 2.9 million acres in the Brazilian rain forest were bulldozed to grow crops that were used to feed farm animal, Imagine now in 2020.
Moreover, plant-based foods are not as satiating as animal foods. Fat and protein are both very satiating. If everyone went vegan, people would be eating more often. This is because of the way the hormone insulin works. A high carb diet sends blood sugar up, so insulin is released to bring it back down. This creates a blood sugar yo-yo effect that leads to frequent meals because when blood sugar goes low, the brain wants to ingest food to bring it back up.
But this yo-yo effect is lessened in a person who eats a higher fat diet because fat barely stimulates insulin at all.
This is a very basic aspect of human physiology that researchers have known about for at least 100 years. Consider the difference between a cut of pork loin and an avocado. They’re both around 200 calories. But the pork loin will fill you up for a lot longer than the avocado will. The avocado is high in fat, but not very high in protein. Now, how many avocados will people have to eat in a day? And where are they getting protein from? Massive soy fields?
So you would have millions of people eating multiple meals per day plus snacks, and frequently eating foods that are not grown locally. Not good for the environment. All of that shipping is going to contribute massively to global warming.
Also, it’s not just cows that release methane. Rice agriculture does too. So even if you get rid of the cows, you’re now more reliant on grains than ever, which means more rice agriculture. Which means you still have a methane problem.
Then, As we already mentioned above, The millions of livestock animals that die off because no one is willing to pay for them to live. The profit motive is what keeps them alive. If everyone went vegan, these livestock species (cows, pigs, chickens) would be extinct or endangered within 100 years. The current stock would all die of starvation.
A vegan diet is a right way for a person to feel good about themselves, and it allows them to pretend that we don’t exist in a food web (something dies for you to eat regardless of whether you eat them directly or not). Still, it’s not a useful environmental tool on a massive scale.
Our soil is in terrible shape already. Depending on it further is not a good idea. What’s needed is more sustainable animal husbandry practices and to stop feeding grain to livestock. Cultured meat at scale will happen eventually too, and that will end factory farming. But it won’t happen overnight.