Do you really think being old, frail, muscle less is going to give you a better life?
Do you really believe protein has a negative correlation with longevity based on some crappy mice studies that over fed their rodents?
I fear for the people that restrict their protein intake and lose muscle tone because they are going to be in for a rude awakening when they can’t lift themselves off the floor when they inevitably fall.
It may depend on the relative amounts of particular amino acids consumed. You probably can limit methionine and promote longevity while still maintaining muscle mass for example. Possibly similar for tryptophan and cysteine.
Eating natural, whole foods, primarily plant based, is the healthiest diet. A diet of beans, nuts, whole grains, vegetables, and some animal products (like eggs, yogurt, or fish) is already very high protein as protein naturally occurs in beans and nuts and animal foods. Additionally, your body needs nutrients and antioxidants from plants.
A diet high in refined foods is still unhealthy even if it’s supplemented with protein shakes & chicken breast.
Protein should be a consideration but is the wrong component to focus on for long term health.