Why is it wrong inherently to cause animals pain when it is necessary for food production. The higher the US output of meat products, the cheaper the prices, and the fewer people who go hungry and more money for other living expenses. Actually, meat consumption to grain consumption is DOWN. More people are getting their calories from grain-based products despite the relatively recent introduction of grains in diet; thus, many people develop diabetes, Alzheimer's (type 3 diabetes) and celiac disease, just to name a few, due to the inefficiency of grain digestion for lack of time in the adaptation of our genes.
Meat, in hunting and gathering societies of the past, constituted 45-65 percent of their daily calories depending on which region we are talking about. It is much more difficult to sustain living on vegetables alone, because of the amount needed in order for sustenance (potatoes and other roots and legumes are not applicable, we only recently began eating those). Therefore, humans have more evolved genetics for meat digestion and plant-based foods health-wise and meat digestion is a viable supplement in a diet for the lack of caloric nutrients from vegetables alone, which is possible, but requires mass consumption and production. Though, not because of animal suffering, we should make reforms in order to prevent antibiotic and hormone use in slaughtered cattle in order to bolster human health and reduce health costs.
Animals eat each other every day. They tear into each others eyes and throat, before moving on to slaughter their victim's family and homes. Are these animals implementing animal cruelty? Yes, they are, but animal cruelty, as far as non-domesticated animals go, is not inherently wrong. Nor is it when a species goes extinct. It is what it is, natural selection; many animals are OVERPOWERED and DEFENSELESS and protecting them is not a necessary discourse, unless it serves man's needs. Man should do whatever is in his power to improve his life and ensure not only his existence but his thriving existence.
Personally, I find the conditions under which animals are killed in nature less abhorrent than the sufferable conditions under which they are kept in factory farms. I don't know how much you've seen of what happens there, but it's unnecessary and cruel. There are animals that are born with a roof over their heads and don't even get to see daylight. The artificial buzzing light is the only sun that exists for many animals that die there, and many of them do die, before even getting to the slaughterhouses, of diseases, of bones that break because they've gone brittle. The average time for a lion to kill a prey is somewhere around 5 minutes, more or less, if I am to believe footage times of documentaries. The average time for a human to kill his prey depends on where you say the killing process begins, which could be more or less debated over. If we say it occurs right from the time we start to hurt the animal, then we can say it happens from birth with a lot of animals, until they're ready to be slaughtered. In any way, it's more cruel than a lion killing a zebra.
While you have said that a high percentage of the paleolithic diet consisted of animal, you've failed to add to this that most of the calories came from seafoods, while the majority of the animals that we see are consumed today in the US are those of mammals and birds, primarily chicken, pork, and beef. Furthermore, the range in percentage of consumption of calories is far more diverse, even in paleolithic times, than you've represented. The Gwi people, who live in Southern Africa consumed only about 25% animal calories in their diet, and some inuit people have a percentage as high as 99% here.
Humans have adapted to their surroundings. People who live in polar regions simply need to eat lots of calories from animals because almost nothing in the form of plant life grows there. The Gwi in Southern Africa had much less animal calories to eat, but the general diet was still healthy. If a European were to have 99% of his calories consist of meat, we'd call him unhealthy, the people up north would disagree.
Meat consumption by itself is alright and natural, but where should we cap it? The Average Chinese used to consume 13 kilogrammes of meat on average in 1982, and the Chinese called beef millionaire's meat, because meat was a luxury product, something for special occasions. Today, it's somewhere up in the 40s, and it's still lower than the USA. Norwegians consume around 55 kilogrammes of meat yearly. Is 90 kilogrammes of meat eating yearly necessary to be healthy as an American? Unlikely. There are many cultures, even today, that do not consume meat in their everyday diet, or people who find their calories from seafood like from clams, shrimp, fish. One diet is not necessarily intrinsically healthier than the other, and it lies deep in the genetic differences between people, and we cannot exactly say that one diet is healthy for all. In fact, you made a very good argument with the grains, and the same argument can be applied for milk drinking, with the majority of the Asian populace being lactose intolerant, but plant-derived works, and there are many ethnic groups that could survive on a healthy diet of "meat once a week is totally fine".