The security budget

Miners spend on hardware and energy roughly what mining pays them. That total, subsidy plus fees, is often called the security budget: the more it is worth, the more an attack costs.

The open question

The subsidy halves every four years. Today it is still the larger part of a miner's income in most blocks. Eventually fees must carry security on their own. Will fees be enough?

• Optimistic view: as Bitcoin becomes more valuable and widely used, demand for block space and its price rise.
• Worried view: if fees stay low, the security budget shrinks, and attacks become cheaper relative to the value secured.
• Middle view: the answer depends on adoption over decades, and the subsidy still has a long tail before it becomes negligible.

Changing the supply schedule to fix it is widely rejected, because the fixed supply is one of Bitcoin's core promises.