As you're on hacker news I assume you're familiar with the trolley problem[1], by factory farming animals we are falling into the same ethical dilemma, we've chosen to pull the lever - but it's not clear in this scenario if we're actually saving lives, this is more of a trolly problem where there are five people tied down on each line, but someone offers you 1000$ to pull the lever and send it down the other line[2]. We are shifting the responsibility for these animal's deaths onto our hands so that we can benefit with yummy meat, so I find the argument above to be immensely weak.

The consideration that animals may become extinct if they aren't useful seems a more worthy one, but less valid in the modern world.

Also, I'm not vegan or vegetarian - I am internally content with my moral and ethical choices regarding meat and have carefully considered them.

[1] If not, see this: https://en.wikipedia.org/wiki/Trolley_problem

[2] Assuming that person's motivations are opaque, if we know they want to save or specifically kill someone then the problem gets miles more complex - if it makes it easier, maybe imagine a piece of china on the track that the train will go down - so your choice is five people or five people and some nice china.