What happens to Bitcoin's security when the block subsidy ends?
The block subsidy halves roughly every four years and reaches ~0 around 2140; today it is 3.125 BTC per block. The intended replacement is transaction fees, which are 0.64% of miner revenue over the last 30 days (as of 2026-09-07). For fees alone to close today’s gap to a 1% target they would need to reach about 18% of miner revenue and hold there — roughly 29× today’s level.
What the record says
Bitcoin’s longest stretch with fees above 20% of miner revenue is 40 days, ending 2018-01-31. The best 30-day window in its history reached 26.2%, in January 2018. Fees have exceeded the subsidy for exactly one day: 2024-04-20, the day Runes launched, at 75.2% of miner revenue (304% of the subsidy). Every one of those episodes came with fees that priced ordinary users out.
The part that does not depend on fees
Hold the best fee month Bitcoin has ever produced — January 2018’s 26.2% — flat forever, and after the ~April 2028 halving the gap is still about $4.71B a year. At today’s fee level it is about $9.16B. The floor is set by the halving schedule, not by the fee market. Details: after the 2028 halving.
The options
Fees rising to that level; changing consensus to reinstate issuance or end the 21 million cap; or an external, market-funded reward credited to miners. The first has never held. The second costs Bitcoin its scarcity promise. The third is what NAT is, and it is bounded by what a market will pay.