Why might transaction fees become more important to miners?

Detailed explanation

Miners earn revenue from the block subsidy and transaction fees. As the subsidy falls after each halving, fees must become relatively more important over the long term. The Bitcoin whitepaper already describes the possibility of funding the incentive entirely from fees after new issuance ends.

This is an intended mechanism, not a guarantee of any particular revenue level. Fees arise when users are willing to pay for scarce block space. The actual amount depends on transaction demand, the fee market, Bitcoin's price and miners' decisions, among other factors. Miners also respond to revenue and costs: if mining becomes less profitable, computing power may leave the network; difficulty later adjusts to the remaining power.

Specialist debate therefore examines whether a future fee market will provide a sufficient security budget. No definitive figure can responsibly be set today. What can be stated is that the protocol allows the transition to fees, but its long-term economics depend on actual network use.