It sounds reasonable to me that this is the situation, except perhaps in very small farms. History has taught me this: whenever humans have physical control over other humans, they tend to abuse their subjects in terrible ways. Slavery. The Holocaust. Gulags. North Korean concentration camps. It seems to me that the way we are treating animals is simply a manifestation of this tendency toward sadism. It is not difficult to find recorded evidence of pure sadism playing out both in small and large farms. Since I have no way to verify a given farmer's claims, I will not take the risk.
Second, a well-balanced, strictly vegetarian diet is far more healthy than a diet that is based on animal products [1,2]. I know that many people do not believe this, and they base their views on the vast amount of disinformation that is out there. To get to the truth you have to listen to the experts. The book [1] I am citing was written by Dr. Walter Willett, one of the leading researchers on the relation between nutrition and disease. His recommendation is basically to eat as I suggested above. More precisely, he recommends (based on decades of research) to reduce animal-based foods as much as possible, and to reduce processed foods in favor of whole foods. There are of course additional recommendations that I will not go into.
[1] http://www.amazon.com/Eat-Drink-Be-Healthy-Harvard/dp/074326...
As a side note, I have a friend who worked in a Buddhist monastery and tended their chickens. He even sang to them. I would totally buy their eggs :)
[1] Louise Burke - Clinical Sports Nutrition
The issue is that you need a well planned veg diet. You can't just eat french fries and white bread and expect to maintain your health, obviously. For me, after a couple months of tracking my food and learning the calories/fat/protein of a lot of plant foods, I don't really have to think hard about creating well-balanced meals. It's a learning process.
I'm sure it's possible to have a healthy diet that includes a very small amount of non-red meat. That small amount is probably not going to hurt you that much. [3] However, you can get every vital nutrient you would get from meat from a plant source without the tacked-on fat and cholesterol.
[1] http://www.huffingtonpost.com/2013/01/30/vegetarians-heart-h...
[2] http://www.nytimes.com/2012/10/28/magazine/the-island-where-...
[3] http://www.health.harvard.edu/blog/study-urges-moderation-in...
50% DV in my soy/almond milk, 40% in a single tbsp of nutritional yeast. I have a cup with cereal in the morning and a cup with dinner at night and I'm set. It's quite easy.
The three main factors that influence health (in the sense above) are smoking habits, diet, and exercise. The factors you mention are secondary. I refer you to the sources I cited above for more details and evidence.
> Basically there is no such one-fits-all scenario for a diet.
That's a bit like saying that not everyone should avoid arsenic... There are of course personal variations, but the fact that meat, dairy and eggs are bad for you is not one of them. The basic mechanisms that cause animal-based food to be harmful, like the fact that saturated fats increase bad cholesterol, are well-studied and do not vary greatly from person to person.
On the other hand, a vegan diet is not one-size; it's not like we just eat lettuce all day. In fact, when you go vegan you discover that you do not lose any diversity, because there are many plant foods that non-vegans usully don't consider eating (for no good reason).