There are clearly other ways to reduce saturated fat intake without resorting to restriciting protein intake, but for this and some of the other reasons discussed here, public health official tend to adopt a narrow viewpoint. There is little if any evidence in humans that high protein intakes are associated with the promotion of renal disease. The data on osteoporosis are also rather clear even if largely misinterpreted. While there is a slight negative correlation of bone density formation with protein intake, and a weak positive correlation with calcium intake, there is a strong correlation with a high calcium to protein ratio. Thus, for a given level of protein intake, a minimum level of calcium intake in required.
More protein means more calcium.
Finally, the calorie restriction, low protein diets extending lifespan have only been demonstrated in small animals with short lifespans living under well controlled laboratory conditions. What is not clear is how applicable these results are to humans free living in a normal environment. Now on to the actual data. Lemon is a respected researcher in the protein requirement field for over 15 years. I'll quote occasionally so that statements I am making are not regarded as my interpretation, but actually reflect what is written.