This is a misleading argument. Yes, miners will always spend more until marginal revenue == marginal cost. The problem with the proof of work system of bitcoin (and many others) is its inefficiency that only allows for a low transactions throughput. Proof of work has no inherent value for the system, it's simply a way to cap the network throughput to about 1MB per 10 minutes because anything lower than 10 minutes will make the network increasingly vulnerable to adversarial attacks [1]. There's got to be a way to increase network throughput beyond this while maintaining censorship resistance and decentralization.

[1] C. Decker, Information Propagation in the Bitcoin Network: http://www.tik.ee.ethz.ch/file/49318d3f56c1d525aabf7fda78b23...