This is simply not true. The common reviled monopoly strategies include price fixing, collusion, natural monopolies, markets with high barriers to entry, network effects, and others in addition to regulatory capture. Regulations that contribute to monopolistic tendencies are "vanishingly small" in number, you just hear about them a lot because people don't like them.
> if this makes GM vehicles more expensive or causes a consumer backlash that damages sales, GM will change their behaviour.
It may be time to walk through history and see all the dark patterns that were solved by regulation. Before the EPA, it was common for manufacturing companies to dump industrial waste directly into rivers, for example.
Consumers only have so much energy for backlash, and your theoretical system requires everyone to have an encyclopedic memory of all the shitty things that every company does and weigh all of that in their minds when making consumer choices. It's impractical and impossible. Regulatory agencies are a much better system and that's why every modern economy depends on them.