How Does a Fee-Based System Ensure That the Cost of a Spam Attack Is High?

A fee-based system ensures a high cost for a spam attack by making the consumption of the scarce resource (block space) directly proportional to the price. To successfully clog the network, an attacker must fill multiple blocks, which requires paying the current competitive fee rate for every byte of data.

Because miners are profit-maximizing, they will only include the spam if it pays the going rate, which can be very high during periods of high demand. This mechanism converts the cost of network disruption into a massive financial burden for the attacker.