The 18 millionth Bitcoin has been mined and the world’s first cryptocurrency is now one step closer to its hard-coded cap of 21 million coins. “The pie is shrinking. This milestone gives people some simple math to raise awareness about where we’re at in the Bitcoin mining process,” said Alex Adelman, CEO of bitcoin rewards platform Lolli, adding: “It’s good for people to see the progress of bitcoin, to look back on everything that has been done and will be done for the next 3 million. … You should pay attention to the next 3 million.” But don’t worry, you’ll have 120 years to do so.

The next 3 million bitcoins will be progressively slower to mine as a result of block reward halvings which occur every 210,000 blocks (or roughly four years) and reduce new bitcoin supply by 50 percent. The final bitcoin is expected to be mined in 2140.

It seems blasphemous even to go there, given bitcoin’s value proposition as digital gold. But outsiders foresee a day when the 21 million cap might, gasp, come up for debate.

Eventually, once there are no more bitcoins left to mint, miners will rely solely on transaction fees, which are paid by users to transfer coins through the blockchain. This change gives cause for concern to some who view bitcoin’s block subsidies as integral to bitcoin’s incentive system.

To skeptics, this could undermine the structure that motivates miners to record validated transactions in the ledger. “All of your assumptions about incentives, risk and value go out the window,” said Angela Walch, a research fellow at the University College London Centre for Blockchain Technologies. “Please take the blinders off and stop assuming that everything will still work well once everything goes to a pure transaction-fees system as opposed to block subsidy.” Currently, with each block, miners get a subsidy of 12.5 newly created BTC, worth roughly $99,370, plus any additional transaction fees, which normally don’t total more than 1 BTC.