I like your math, but 100% or beer drinkers do not buy our product. If I am any indicator, I treat myself to good beer about half the time, exchanging quantity for quality, and I buy a 12 pack of SAM about 1x per week, or one of their assortment packs (LOVED the IPA assortment).
Your math is, however, a reasonable approximation of the potential size of the market. Try 300 million people in US, 20% beer drinkers, 20% buy a SAM 12 pack each week for $12: that is a $7.5 billion market in the US. This would be about 10+x SAM's current sales, and would still be a rounding error on BUD's balance sheet.
Our aging demographic is probably a good market supporting trend for my quantity for quality exchange. (I am 48).