> Plants require far more effort for the same amount of energy.
I'm confused as to what you mean here. Do you mean the energy required to produce plant foods is higher than the energy required to produce animal foods? If so, this seems self-evidently false: all edible calories start out as plant calories, and we can either eat the plants or have animals eat the plants (wasting a bunch of the input energy along the way) and then eat the animals. Or do you mean the effort to prepare the plant foods is higher? If so... maybe? But that'll inherently be subjective and depend on what people want to eat. Certainly, meat-free prepared foods are not difficult to find, at least in the US.
> But no human society throughout human existence and pre-human existence were vegan because plant diet isn't optimal... Balanced diet folks. It's something humans have been designed to eat since human existence.
I don't think many vegans argue that humans were designed to eat only plants (many would dispute that humans were designed at all), but looking at historical societies isn't that useful; advances in agricultural technology have made eating a healthy vegan diet more feasible than it used to be, so the fact that it wasn't practical to do 100 years ago or 1000 years ago or whatever isn't very interesting. In the here and now, it is possible, and there's plenty of peer-reviewed research suggesting that the optimal amount of red meat, in particular, to consume, is none (it's carcinogenic), that animal fat of any kind isn't great either, and that people who reduce or eliminate consumption of these foods while still maintaining adequate caloric and nutrient intake have reduced all-cause mortality.
In any event, many vegans would also argue that even if it wasn't possible to eat vegan historically, the fact that it is now means that we should, for reasons of ethics/animal welfare, greenhouse gas emissions, etc.
> It's also why vegans tend to be lethargic, lose bone mass, teeth rot, joint pains, have trouble concentrating, etc.
Citation needed?
> It's why vegans have to take tons of supplements to even stay alive and while doing so do long term damage to their bodies.
The only nutrient that only occurs naturally in animal sources is B12. It's true that vegans generally need to get their B12 either from supplements or fortified foods, but the technology for that exists, so there's no real downside here.
Other than that, many vegans choose to get other nutrients from supplements or fortified foods because it's easier than planning a diet that includes all of them naturally. Omnivores do this too, though; we just don't think about it, because that diet is standard enough that it makes sense from a public health perspective to incorporate nutrient supplementation directly into the regular food supply so people don't have to think about it. That's why there's iodine in table salt, or vitamin D in milk. If everyone were vegan, we would do the same for nutrients that require special attention in vegan diets, but that says nothing inherent about veganism.
And I have no idea what that "long term damage" claim is about. There's nothing inherently dangerous about getting a nutrient in a capsule instead of in a steak.
Your link also suggests that having healthy gut flora is important (true), but doesn't support the case that you need to eat animals to achieve it, at least as far as I can tell.