A scalable bitcoin ends up in a dozen data centers. The cost to set up such data centers is few hundred millions plus tens of millions in yearly operations. Miners must secure their infrastructure uptime to remain profitable. There is huge risk and little reward for any such mining company to act dishonestly on new blocks or break antitrust laws. Also it is easier for governments to audit a few large publicly traded miners than auditing thousands of small and inefficient miners. The nature of the bitcoin protocol security is economic.