I'm not disagreeing Gnarlie, but I think it's an interesting observation that you make: I've never heard anybody express it the way you have.
If I understand you correctly, you're expressing the viewpoint that the term "black belt" literally has a different definition for each person. Like, if I'm an 80-year old student testing for 1st Dan, the definition of "black belt" is: "Have you performed at the level we would expect a dedicated, hard-working, focused 80-year old person to perform at." Or as another example, if a person weighs 400 pounds and tests for 1st Dan, the definition for them would be, "Have they performed at the level we would expect from a dedicated, hard-working, focused -- but overweight -- 400-pound person."
Though I've never thought of it this way before, I find that viewpoint interesting. Black Belt then wouldn't be a measure of proficiency as much as it would be a measure of focus, dedication, and hard-work. It makes sense, but it's very different from the "Could you hold your own in a bar-fight against multiple opponents" definition that we so often fall back upon in these discussions.
Or to put it another way, and I know I'm over-simplifying, but if I understand you correctly, your definition of Black Belt is tied more to effort than to proficiency. Like, it's not "are you good", it's "are you the best you can be." That's food for thought.