A simple thought experiment can show that if Satoshi had kept all 21 million Bitcoins to himself and never shared this idea with anyone, the value of Bitcoin would be zero.
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1- So go and buy all 21 million bitcoin from satoshi for zero in this scenario. i'm sure he won't give them away at this price
2- satoshi never had 21 million bitcoins. When wrote his last post on bitcointalk (when he disappeared from crypto space), there were only 20% of bitcoins mined.