While they didn't know about microbes, they understood that drinking dirty water made them sick. Beer gave them the fluids and did not make them sick. The paraphrased saying was common in the first millennium "drink beer, water is poison".
So a tribe would not survive until they brewed massive amounts of beer? That doesn't sound like that would work out.
You would also have to buy it. How did the poor city folk get on?
I don't think its possible that you would have to wait on beer to be brewed in huge quantities for a city to grow. Here is a non definitive view on it,
People drink contaminated water to this day. It wasn't a claim that the only thing that was drank was beer. It was a historical fact that beer has been a common beverage since the Egyptians and wine has been a common beverage since the middle ages.
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