Bitcoin miners are paid two ways: newly issued coins, and transaction fees. The issuance halves roughly every four years by design, and will eventually approach zero.
The intended outcome is that fees replace issuance. Whether they will is genuinely unsettled.
The current position
Issuance still dominates. Fees contribute a minority of miner revenue in most periods, spiking during congestion and falling back.
Each halving cuts revenue immediately while costs stay the same, which forces consolidation. Operations with expensive power exit, efficient ones absorb their share, and the network continues at a lower total cost.
Why security is tied to this
The cost of attacking the network scales with what miners spend. If miner revenue falls substantially and permanently, the cost of assembling enough capacity to attack falls with it.
That does not mean an attack becomes likely – it means the margin narrows. The margin has been very wide, so narrowing is not immediately alarming.
The fee market question
For fees to replace issuance, either transaction volume must rise substantially or fees per transaction must rise substantially.
Block space is fixed, so volume cannot grow much on the main chain. That leaves higher fees, which pushes ordinary transactions to secondary layers, which reduces main-chain volume. The dynamic is circular and nobody has resolved it on paper.
What the network has done so far
Adapted, through several halvings, each of which was predicted to be destabilising and was not. That is evidence, though it is evidence from a period when issuance still dominated.
The honest summary
A structural question with a long time horizon and no consensus answer. Positions on it correlate suspiciously well with whether the person holds any.