An argument so tired it's unconscious

I spotted this in a comment thread.  This is argument is a tired bit of nonsense, but I’ve never seen it argued with such verbosity.

To anyone saying we should all become vegans because of the inhuman way animals are being treated. You are not thinking things through. Say we all become vegans. Eventually farms all over the world would go out of business. And do you think they would just let all the cows and chickens or all livestock go free to roam where ever they please? No, you would be condemning millions of animals to a needless death because their would be no use for them. Are you going to take the 500 head of cattle at the farm down the street and take care of them? Are you gooding to make sure they are kept healthy and well fed? I think not. Becoming vegan is not the answer to the inhumane treatment of animals.

Of course the problem with this argument is the premise that everybody will become vegan at about the same time.  I don’t think any person can seriously say that is going to happen. Over the last 15 years, the number of vegans in the US have gone from less than 1% to about 2.5%.  Let’s assume an outlandish scenario: that this continues to double each year.  Given that, it will take about 55 years for everyone to become vegan.  I’m sure everyone would agree that this is plenty of time for supply and demand to take care of things without widespread panic.

But given that faulty premise, the logic is correct. If, tomorrow morning, every single person woke up and decided to be vegan, we would have billions of enslaved animals that would need to be dealt with.  It’s plausible that, in that scenario, countless animals would be left to wander across the land.

But wait!  This is a fantastic premise for a post apocalyptic sci-fi novel!  Get this: An advanced alien race comes to earth, and seeing our violent tendencies bestow upon us what they think is a tremendous gift:  they manipulate everyone’s brain so that they are now vegans and pacifists.  Suddenly the world is a peaceful place, no person ever harms another person or animal.  But the farmers who, in their shame about what they had done to the animals, open the gates and give them their freedom.  Billions of cows, pigs and chickens spread upon the landscape in ever increasing numbers.  The highways come to a standstill due to wandering animals, the streets awash in their excrement, contaminating rivers and ground water.  People begin dying of cholera and other new zoonotic diseases.  The rag-tag remnants of humanity abandon the now overwhelmed landscape, drifting through the oceans in derelict cruise ships.

But the alien’s manipulation is not perfect.  Eventually, mutations occur which weakens the changes.  The selfish gene, indeed.  Centuries pass. Our story follows a teenage girl who has visions of the circle of life, and that we are really supposed to be fierce predators (look at these canines!)   She begins a new religion which preaches this and advocates for respecting the animals’ spirits by liberating them from their earthly bonds (by stabbing them) and eating their corpses.  This becomes a mass movement.  The animals are, once again, confined, enslaved and killed in increasing numbers.  People retake the land from the marauding beasts, and all is right in the world again.

We’ll write this for a young adult audience, a cautionary tale about the dangers of veganism!  It’ll be a bestseller!