I'm not going into why the term omnivore doesn't really mean anything, cause if we are basically every animal is.

I don't know about the "vegans" you know, but i doubt any true vegan (which is a state of mind, not a plant-based eater, that's just a consequence) would go back to paying people to enslave, rape and murder animals for their consumption.

But the rest you say, the same is said by ignorant statists about anarchy.
perhaps morality won't even be a part of it, which in the beginning is the case of a lot of eventual vegans by the way.
But the world will go plant based because of the unsustainability of meat production and because of the science of nutrition/health.
Just like it is necessary for anarchy, people have to get out of the state of fear and ignorance and get into a state of love, of acceptance of truth, rather then ignoring it, if this doesn't happen at all we won't get anarchy to begin with and we as a species will eventually drive of a cliff.
For anything to change, the realm of causation (the minds of people) needs to change for the realm of effect (the world around us) to change. There is no change to be made in the realm of effect.

For the unsustainability part there could become technological devices to cheaply produce muscle tissue in lab-like circumstances, where no animal after the initial biopsy needs to be involved in, but when the science on healthy nutrition gets accepted i doubt people would feed their children meat and they themselves might perhaps still eat that petri dish meat out of addiction, but even if humans continue to eat meat, meat grown in a petri dish is reasonably vegan friendly.