Being a billionaire is so extremely rare so it doesn’t matter.
Social mobility is the ability to move from lower to middle or upper class not from lower to the extreme upper class.
Then define your terms, what is the mobility method, what are the bounds of the classes and by what metric and, what is excellent/great/good/average/poor/bad/terrible mobility?
Oh, you had to be a billionaire to be middle class? I never knew that.
If there’s no way to get to the top, how is the upward mobility “great”?
Being a billionaire is so extremely rare so it doesn’t matter. Social mobility is the ability to move from lower to middle or upper class not from lower to the extreme upper class.
Then define your terms, what is the mobility method, what are the bounds of the classes and by what metric and, what is excellent/great/good/average/poor/bad/terrible mobility?