regarding how many people will or can reach higher belts,
you can see this by the belts achievements:
4324012 people played 1 game to get white belt,from those 4 million only
753356 people played 20 games to get yellow belt,
31213 people played 1000 games to get black belt,just a mere
1781 people played 10k games to get 10th dan,833 ppl got master
and 579 got to custom
the worst part apparently are the 4 million players that abandoned the game
before even playing 20 games,but lets say they didn't understand the game,
they got hyped,they got bored etc.
in truth the worst part is later were you see dedicated players,that played
the game and they know the game,quitting.
so 2 major problems,the newly attracted players never start playing
and the older dedicated players stop after awhile for various reasons