We are already past the half way mark (11mil+ already mined). This means that we can only mine 10 million in the next 128 years.
Am I missing something? That doesn't sound right.
And yet it is right.
As you get closer to the last coin, the difficulty increases exponentially - meaning you'd need virtually infinite computer power to solve it. Add to that, the reward for finding a block reduces by half every x number of years. By the time we get even remotely close to mining the last coins, it won't be worth the power required.
You are correct about the fact that the block subsidy "reduces by half" (approximately every 4 years). The rest of it you have some learning to do.
The year 2140 is only based on the software's current precision, which divides bitcoins up to 1/100,000,000th. This would imply that around year 2140, the block reward would become zero. However, long before 2140, the precision will be increased (to support smaller fractions of bitcoins, or floating point or whatever) so mining can continue forever.
There is no guarantee that this will happen. It might, but it very well might not.
Mining is never "done" and the actual 21 million will never be reached (although it comes arbitrarily close).
Mining is never done because it will be supported by transaction fees. In the current design the exact amount of bitcoins created can be calculated, it will be exactly: 20999999.97690000 BTC
Due to the accidental permanent destruction of a few bitcoins by a bug in some mining software, the total unspent outputs in the blockchain will actually be a bit less than that.
But guys... that is VERY bad. It technically speaking means that in our lifetime we'll only see around 15 mil (very wild guestimate) in our lifetimes. The last 5 million coins wil take FOREVER to mine.
Not forever. 15 million will be mined before the end of 2017 (how old are you? I expect to see 20 million mined within my lifetime, in the next 15 years). It will about 136 years from now until the last bitcoin is mined based on the current implementation. Why would that be bad?
