Here's the thing about trail ratings. They are about difficulty. Difficulty is a subjective thing. Difficulty isn't just slope angle and trail width. It has everything to do with how a certain run skis, and that doesn't always translate into numbers. In fact, it rarely does. So the way trails are rated is to ski them, and say 'this trail is more difficult than that trail, which is more difficult than that other one'. Its completely subjective, and based on knowledge of the terrain, not numbers.
In order to establish a universal system, it would have to be based upon a single subjective scale. Which means one person would have to rate every single trail's difficulty in the entire country. That's a process that would take decades. There are almost 500 ski areas in the USA, and most ski areas would take multiple days to ski all of the trails. Then they would be rating based upon their memory of thousands of other trails, some of which they skied many years ago, and just once. Do you expect somebody to say "This trail I just skied is more difficult than a trail I skied once 8 years ago, and have skied a couple thousand trails since then?" Of course not. Its not even close to realistic. Of course, over the time span of rating, that person is going to age. And suddenly the trail he rated as a blue when he was 24 would get rated as a black when he's 45. Then after his career is over, then what? No more trails allowed, because our trail rater is retired?
Your whole argument is based on the idea that trail ratings are based on numbers. They're not. They're based on difficulty. Numbers play into it, but they're not the determining factor. Trail ratings are now, and will always be subjective. Subjectivity is not something you can standardize. So really, give it up.