The perspective of both sports medicine and military practice is that they must show real, practical results.
For sports medicine, a mere study is not sufficient...professional athletes must actually win. Soldiers must actually perform better in the field.
For my own experience, I've worked out all my life, and showed little to speak of...until in just these later five years I've seriously met the recommended protein levels and for the first time see results I'd have been happy to see in my 20s.
This interview might interest you. Dr. Hamilton Roschel is a research physiologist at the University of São Paulo, and Chris MacAskill is an Earth scientist and triathlete. Halfway through the interview, Chris asks Dr. Roschel about a study he did that found that protein supplementation (to 1.2g/kg) had no additional effect on frail elderly people otherwise consuming the RDA for protein. Only resistance training was effective. According to Dr. Roschel, people that are elderly and frail have anabolic resistance, and have a very diminished response to protein stimulus. It's also difficult to treat, even if more protein would theoretically be better, the amount of protein that might be effective (above 1.6g/kg) is difficult to fit within a palatable diet.
The most pertinent part of the interview is at 25:00
One thing that also seems obvious to me, that wasn't really covered, is the economics and environmental impact of a general recommendation for higher protein consumption in the elderly. Protein is potentially a very expensive macronutrient, both in terms of capitalist economics, and environmental impacts. Some countries like China are actively trying to limit the consumption of meat for the purposes of meeting environmental policy goals, and while there is general agreement that this would be beneficial in Europe among scientists, it is is considered politically unacceptable at the moment. Insect protein economics would be more favorable, but there is a big problem in Europe and the US with the acceptability of insect protein as human food.
I'd be curious to find research on elderly people eating a plant-based diet. I am still looking into it. My own hypothesis is that lower renal acid load might have a positive effect on muscle protein synthesis, but that is just a hypothesis. Anecdotally, there have been athletes who ate exclusively or predominantly plant-based diets, who retained substantial muscle mass even into their 80's and 90's (Manohar Aich, Joer Rollino, Jack LaLanne, etc.).
I also wonder about the role of atheroschlerosis in frailty. Good blood flow is important for health in general, it seems. In Ikaria and Sardinia, two of the so-called "Blue Zones", many of the long-lived elderly have good cardiovascular function and nitrate oxide production, and still live relatively active lives even into quite advanced age. Whereas in North America and northern Europe, some degree of atheroschlerosis is almost unviersal in the population, even at a young age (I was told nearly a decade ago I had some coronary artery blockage, for instance... it's pretty much unavoidable if you eat a standard American diet).