I do not agree with your data because when you do 21M/7B is the wrong equation, and i say this because the last bitcoin will be mined in 2140, and when that happens the world population will not be 7 billion, it should be bigger than that.
Well, it's not meant to be super accurate. I'd rather work with a round number. Notice that 21/7 is 3. I could adjust for bitcoin in circulation and an accurate population number but that is meaningless. Close enough is good enough in this case.
I see your point about using more accurate numbers, it would be fairer to assume the usable number of bitcoins is lower. However, most bitcoins will have been mined long before 2140.
I think that trying to round the numbers to be more accurate at this point is meaningless.
We could argue that the population is actually 7.5 billion but 30% of it is under 18, cutting 2.1 billion, but again in some countries, the 18 yo age has no significance, throw in the fact that some bitcoin are lost forever, probably a few have been lost just because the owners had little dust left like 1000-2000 sat in their wallets and decided it's not worth to keep it with a higher fee than value and so on and so on....
You've made the mistake of using the term "fair share" and that made a lot of people assume the topic is about something else.
As long as workers don't own the means of production there will always be great inequality. Even if you divided all the bitcoins evenly among everyone in the world the same wealth inequality would happen over a short amount of time due to owning assets like companies.
Let me tell you what happens when the workers own everything, cause I've lived in such a society.
Indeed, we are all equal, we all lack food, heating, hot water, everything.
We all become equally dirt poor, staying for hours in line to get a litre of milk, one of oil and 250gr of butter.
So the moment we start dividing those equally, I have the feeling they will be worth zero.