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Your math is off by 1,000x.
"2.1 × 10^15 sats ÷ 20,000 sats/person = 105 million users"
should be
"2.1 × 10^15 sats ÷ 20,000 sats/person = 105 billion users"
So based on your framework bitcoin scales to 5-10 billion people.