According to Blockworks, the Bitcoin network experiences a halving of supply approximately every four years, with the next halving set to occur in 2140. Satoshi Nakamoto, Bitcoin's pseudonymous creator, programmed the year 2140 as the time when block rewards would drop to less than one satoshi, the smallest unit of bitcoin. The halving is modeled after the physical mining of scarce resources, according to Jameson Lopp, co-founder and chief security officer at Casa. He notes that Nakamoto explained the supply schedule to Mike Hearn, a former Google engineer who became an early Bitcoin contributor, in 2009.
Satoshi chose 21 million as the number of coins and distribution schedule, but the divisibility of a bitcoin could be represented differently depending on its value. As the rate of issuance declines for 132 years, transaction fees will become more important for maintaining the network's hashrate and security. Lopp is optimistic about the future of the multi-layered Bitcoin ecosystem, expecting on-chain transactions to pay relatively high fees as the base chain starts to resemble a high-value cryptographic accumulator.