Economic incentives can become mal-incentives. Once a mal-incentive exists long enough, entire industries start depending on it. At that point fixing the problem threatens the people profiting from the distortion. The incentive system becomes self-defending. I hope you can see the irony. If you don’t, I can give you some real live examples to prove incentive can be co-opted just as easily and then be turned on its head. See Goodheart’s Law for reference. And substitute “the measures” for “miner revenue”, “security budget” or whatever made up metric you like to justify miners mining spam.