Excellent point of clarification. On another note, shouldn’t we all decide to be plants?
Would that make it wrong to wear a fir coat?
Leaf me alone!
I prefer to leave a crumb behind to see if anyone picks it up. (Nobody picked up this one!)
More like “why do bank robbers exist if we could just all agree to be exotherms, therefore needing less food, heating, and therefore money?”
I’d just like to say that I hope in future OP does get the opportunity to elope. Everyone deserves happiness.
Maybe it would be useful to address the steps of the OP part by part?
Whether or not all animals can be vegetarian is addressed well already, and even to the degree that many can, what vegetables they can digest are limited. The pebble of truth though - within not clearly determined species specific limits gut microbiomes do adjust to different diets over time and are, to some degree passed down mainly from mother to child.
An animal does not have to learn that pain hurts. That’s tautological. Avoiding pain unless the payoff clearly is expected to offset it is also not learned but innate.
As pointed out, being a vegetarian/prey does not avoid confrontation; it merely defines which side of the confrontation you are more likely to be on.
Pretty sure that herbivores in general die more frequently as the result of confrontations (in which they are prey) than do carnivores and omnivores. But then again I am not so sure. Lots of confrontations are intra-specific, over territory and mating access. Any input from our animal experts on whether or not those battles that may result in death are more common for one or the other group?
In general the massive saurian herbivores had very tiny brains. Velociraptors on the other hand were relatively large brained.
It does raise an interesting question - how many of the smarter species are herbivores? And there are a few anyway. Many apes are at least primarily herbivorous. Elephants.
Most of our primate ancestors were primarily vegetarian. So why DID humans start eating so much meat?
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Okay here it does not follow. A discussion about what counts as “culture” and which animals have it to what levels could be interesting, but clearly diets primarily herbivorous, carnivorous, or omnivorous , preceded “culture”.
And culture is itself not a conscious choice that individuals make. It exists to no small degree of learned behaviors that are passed on because the groups that practice those behaviors thrived better than those around them that did not practice those learned behaviors.
There used to be a show on Animal Planet called Animal Cops where the Humane Society and local police teamed up to deal with animals in horrible situations. There was one show with a lady who was a vegetarian and she didn’t seem totally with it, but she was feeding her cats a vegetarian diet. They were all blind,
Offhand, I would say carnivores, particularly large ones, are far more likely to kill one another in territorial or mating confrontations, simply because they are designed to be killing machines. Herbivores usually fight among themselves with horns or antlers. While it is possible to gore an opponent, the fights are usually ritualized to strike horns against one another without doing lethal damage.
But let’s not lose sight of the fact that “primarily” need not, and in this case does not, mean “exclusively.” As the article notes, even going back to our common ancestry with other existing great apes species, we come from a line of omnivores. It’s not a coincidence that Chimps, too, enjoy meat from time to time.
So the article is a bit click-baitey in the regard. More so than I’m used to for The Atlantic.
Yet, one could argue that the very fact that carnivores kill each other so much more easily makes the development of ritualized displays more important. The best I can find so far myself is this bit applying game theory to predicting the development of non-injurious fighting displays. An individual that can easily die in a fight for resource that can effectively signal quitting when it is clear that the odds of survival are poor enough to not offset the value of the resource, will be selected for.
But then I see this about lions and think you must be right!
Well it also notes that even herbivores are not always strictly so.
Speaking as someone who just enjoyed a wonderful half slab of ribs last night with no guilt pangs, I do appreciate the significance of climate change as a driver - to meat as a food choice in those ancient days, and now the impact of our eating so much of as a contributing factor to climate change … there is some sick poetry there.
I see what you did there. 
That deserves a major golf clap: discreet but heartfelt.
Seems to be similar to or a subset of something known as signalling theory, which is not just limited to interactions between predators, and can include (as the foremost example of a “stotting” springbok indicates) signaling of a sort between predators and potential prey. Not that they necessarily know what they are doing.
Accepting the OP’s premise for the sake of argument, how would this work out for fish? Let’s say fish stopped eating other fish (or more broadly sea animals stopped eating other sea animals). Is there enough plant life in the oceans to support the dietary needs of all of the animals on a sustainable basis? Or would the animals eat all the plants and then starve to death?
And don’t bother making the obvious joke. My username is based on the Winsor McCay character.
Unlike on land, where most plants are relatively large organisms, except in near coastal environments most of the photosynthetic organisms are microscopic algae or cyanobacteria. These are mostly eaten by very small zooplankton, although some larger organisms can harvest them. So there’s plenty of plant life, but the majority of animals couldn’t consume it. They have to eat smaller organisms that eat it.
If cantelope hasn’t already fled after all this flogging, I’d like to say Welcome To The Straight Dope Message Board!
Boy, this brings back memories! I made my first post a long time ago. The subject was Gatorade. I made a handful of factual mistakes, but my biggest mistake was not looking stuff up before posting it, so I’d have some references to cite. Some of the regulars boxed my ears and cuffed me around a bit, but I didn’t give up. I admitted my mistakes, and I clarified my solid points.
Over time, I learned to not (usually) assert things I wasn’t sure of. I also learned to be vigorously skeptical in the world outside the SDMB.
I learned to say, “Wait, how do you know that to be true?”
You can be a vegetarian here and get along with folks, if you’re not a jerk about it.
Aw, crud, I’m rattling on like I think I’m wise or something. Sorry.
There is some suspicion all is not as it seems with our newbie OP. I’ll not say more in this context.
Thanks, @Darren_Garrison. That was great!
My cats are second-hand vegetarians. They only eat animals that don’t eat meat.