> There's probably some economical theory around this that I don't know ... competition is good for the consumer when ...
Yes, there is a pretty standard economic proof that much of capitalism is based on. It says that an economy is most "efficient" (you can take a class on that word), when three pre-requisites are met:
1. Perfect information: Everyone knows the value of everything.
2. Commodities: Products created by competitors are equivalent. This works well for oil and bananas, but not so well for much else.
3. Access to Capital: This means new competitors can start competitors and enter markets easily. This isn't so easy when there is regulation, monopolies, or network effects.
In short, there is a set of circumstances when things work really well. However, in the real world, those circumstances rarely arise.