If it will help dying several years later is something one will never be able to prove.
Prove - like make into a fact? No. but
Prove like a hypothesis? A theory? of course.
and there is already so much "proof". There are huge population studies like The China Study, The Okinawa Study, and the Adventist study. They all pretty much "prove" the same thing. People get sick less often and live longer if they eat less meat.
Then you have hundreds of little studies. Like when a researcher checks the blood pressure of ten men. or tracks 100 women who have had breast cancer.
Then there is what I'm going to call reverse studies. Like when they associate red meat with colon cancer. so one can assume that eating less red meat is good for you.
In the end a diet is always a compromise. Even if there was "a best diet ever" it would be worth zilch if basically no one is able to follow it longterm. So I rather follow a diet that is manageable for me.
Of course, a diet is not worth zilch if you are not on it. Like what trainers say, " the best exercise is useless if you don't do it". So there is a lot to be said about finding an exercise you like. And it's the same with diet.
But the motivation factor is very important. We have all heard or read the stories of the guy coming into the doctor's office and hearing, "If you don't start taking care of yourself, you will be dead in a year". and then the guy loses 50 pounds. People are way motivated when it's literally life or death.
I think people vary in how they translate nutrition info into behavior. Some people will go on all kinds of diets and exercise plans if they have heard it might add 5 years to their life. Others won't even make a small change even when they Know it will add 10 years to their life.