What would happen if Earth became a planet of vegans?

Killing

Animals would have continued to compete for territory, females, and resources, so bloody battles, often fatal, would have gone nowhere.

Metabolism

The blood of many animals of the “vegan planet” would not have been built on hemoglobin. And it should be understood that iron of “animal” origin representatives of fauna assimilate much better than “vegetable”.

Diseases

If there were no predators and scavengers, the number of populations would be regulated by diseases.

Homo sapiens

And there would be no place for it on a planet like this. It was meat-eating and the need to hunt and kill food that led to the development of the brain of higher primates.