Because the difficulty changes, giving you less and less bitcoin until the difficulty is so high that your mining computer is obsolete. People used to mine 100s of bitcoins per day with a GPU that only mined at 100-200 kilohash per second. Today that $12k mining rig (that hasnt even shipped yet) mines at 3 terahash per second and mines less that 1 bitcoin per day. By the time it does ship, it will likely only mine .1 bitcoins per day if the difficulty keeps increasing like it has been.
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Yeah I get the whole 'harder to mine' thing but really that's more like inflation. I can say that my house will be worth 10 million in the future because the price of everything rises with time. Any they don't make any more real estate.
The last ever bitcoin may cost $100K to mine but that doesn't make all bitcoins worth $100K. You have to average the total cost of production over the total number in circulation. Also, while their is a limit on the number of bitcoins in circulation, there is no limit on similar and possibly better cyrptocurrencies becoming available and providing unlimited supply.
Plus if electricity prices fall dramatically, bitcoins will be cheaper to make than they are today.
