I think that scenario would have the Law of Unintended Consequences running wild.
In a weird way an argument like that is interesting -- there's something obviously wrong with it, but it's hard to clearly point out what. (It's interesting that the 'rhetorical strength' of an argument may have little to do with how true (or wise, in this case) it is.)
What gets me is the glib nature of it -- "we'll just genetically re-engineer carnivours to be herbivors. It'll be a cinch! And what could possibly go wrong?"
I.e. Say we re-made a tiger to prefer grass. Ok, so it's brain now has an instinct to eat grass, but it's teeth are the wrong shape. And it's gut can't process vegetable matter, so we have to re-make those 2 things, too. And it's hundreds (thousands?) of metabolic process are all wrong for that, so we'll just fix those up too in some unspecified way. And maybe it's muscle mass is not supportable on a non-protien diet, so then what? And on and on. In the end it's not even going to be recognizable as a tiger. It'll just be some miserable mutant monster. (and probably uglier than the ones on the Island of Dr Moreu)
I remember back in the 80's someone famous (Salk?) was going to make an HIV vaccine, 'no sweat, no problemo.' And now, after 30 years of fail, still nothing. I guess there was something he didn't expect.
And that's the thing, it sounds like the author/founder of the Abolitionist Project doesn't really appreciate complexity, or real science, or have a lot of common sense. Sure, we can mess with genes in some elementary way, the same as we can make vaccines. But it's not like in Sci Fi -- it doesn't function in a way that the 'mythological' part of out brains would want to have it function. Lions laying with lambs is a nice mythological image, but not a workable reality.
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Aspie Quiz: 160/43
Alien Quiz: √2/pi