What Is the Economic Rationale for a Miner to Always Prioritize Fee-Paying Transactions?

The economic rationale is based on rational self-interest and profit maximization. Miners invest significant capital in hardware and electricity, making them profit-seeking entities.

Since block space is a limited resource, a miner's best strategy is to fill their block template with transactions that yield the highest fee-per-byte. Including a zero-fee transaction represents an opportunity cost, as that space could have been used for a profitable transaction.

Prioritizing fee-paying transactions ensures the miner achieves the maximum possible immediate revenue from the block they successfully mine.