1. Yes, the foods to which we are most well-adapted constitute precisely the optimum diet for human beings. Are you suggesting that there are foods to which we are ill-adapted that are better for us?
2. Agrarians have settlements that allow the population to swell exponentially beyond what a hunter-gatherer tribe would have been. There is strength in numbers. There is also more specialization of labour in settlements, which means more people to devote their time to making technologically advanced pointy things to kill off competition for resources. Agrarians have technology on their side, which is what helped them win out over the hunter-gatherers. The PB is about the health of the body and mind, not about giving up civilization and advanced technology.
You lousy kids! Get off my savannah!