I don't think it oversimplifies it at all. You need to eat less than you burn to lose weight. If you eat tons and tons of meat every day that's a sure fire way to set yourself up for high cholesterol and a heart attack or stroke.
Some of our Ice Age ancestors (if your ancestos were in Europe) didn't eat very much vegetation because it wasn't available in big quantities on the wind blown steppes. However eating the way they did will lead to an early death. It didn't matter so much for them. They had children at young ages and lived long enough to pass on their lifeways and wisdom to the next generation, but by today's standards their lives were very short.
Fatness was valued (look at the Venus figures) because it meant surviving through times when less food was available.
I think today when we are in a state of constant plenty, avoiding fatness (which goes against our instincts) is the key to survival. Taking advantage of the phytonutrients, fiber, and vitamins and minerals found in vegetables and fruits makes sense.
Every major civilization has a carb based diet, and most of these carbs are grains. Grains didn't pop out of nowhere. Even our remote ancestors ate grass seeds sometimes, just not in the quantities we do today, and since the ice age, European people and those in the Middle East in particular have eaten a LOT of grain, and mostly, it benefited society. Whole grains also have a lot of minerals, antioxidants, phytonutrients, and fiber. To say we're not evolved to eat grain is taking a skewed view of evolution. Humans have always eaten seeds. Grass seeds are just another type of seed that was consumed in the distant past too. Refined grains aren't very good for you but whole grains are fine.
Legumes aren't some abomination. They're also healthy for people. I have no idea why paleo eaters are against legumes.