The 21 million actually derive from a few other educated guesses, which may be a bit easier to relate to:
- 10 minutes between blocks (network propagation delay doesn't allow this to be much lower)
- 50 BTC per block (that's probably the most arbitrary)
- 4 years between block reward halving (that might come from expectations about the growth-rate of Bitcoin usage)
The 4 years were rounded down to 210.000 blocks probably just to have nicer numbers.
Pity he didn't chose 100 BTC per block to start with, because then we'd have a total of 42 million BTC
- 10 minutes between blocks (network propagation delay doesn't allow this to be much lower)
- 50 BTC per block (that's probably the most arbitrary)
- 4 years between block reward halving (that might come from expectations about the growth-rate of Bitcoin usage)
The 4 years were rounded down to 210.000 blocks probably just to have nicer numbers.
Pity he didn't chose 100 BTC per block to start with, because then we'd have a total of 42 million BTC
I also don't understand the reasoning. If you're serious about a currency becoming a major player in the world, you'd at least try to keep the expected single unit value in the same order of magnitude as existing currencies.
That's not going to work because Bitcoin price is so highly volatile. Even an order of magnitude can be crossed quickly within weeks as we have seen. If you design it around the market capitalization of the whole world economy (or only the part which is accounted in USD) then we'd have to deal with immensely large numbers now or the inflationary phase of Bitcoin would be much much longer.