They can probably make Coors sell better than Miller, or vice versa, OK. But that's a minor adjustment, even if worth a lot of money, not a massive change in behaviour like stopping half the population from discovering readily available video games.
It's not minor. What you should be considering is how many of the coors+miller drinkers would have drunk other things without the blanket advertising.
A better example: Coke vs Pepsi. Two big brands. Lots of people will claim they choose whichever they drink without advertising making their mind up for them. Yet there are plenty of other colas out there, with very similar tastes, that hardly anyone drinks. This is low technology stuff, sugar water, it's not that hard to make a cola drink taste good. The reason why people only buy the big brands? Marketing.
It's worth pointing out that "marketing" includes such things as distribution and availability. If I really liked RC Cola, I would be disappointed often. But every corner store, fast food joint, and taco truck has at least Coke or Pepsi and often both.
"Yet there are plenty of other colas out there, with very similar tastes, that hardly anyone drinks."
This also applies outside colas; lemonade, limeade, fruit juices, bottled water, sports drinks (non carbonated non cola sodas). Cokes enemy isn't pepsi so much as tapwater.
