It's important to understand that Bitcoin is now released into the wild. It is now totally supported by the free market. And it's a voluntary currency.
Even the creator(s) of Bitcoin whover that is (or are) can't stop Bitcoin or change it now. Bitcoin is simply governed by all the people that use it, and computers check that everyone is following the same rules using impartial and indisputable math.
As for why pick 21 million coins? You're right. That's an arbitrary number. It could be more or less. It doesn't matter. The only thing that matters is that people KNOW how many there are so they can assign value to them accordingly (and that this number can't be changed/inflated).
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