re: if NOBODY buys a gallon of gasoline from Exxon-Mobile
This "urban legend" has been going on since at least 2001, according to Snopes:
Gas Out
Claim: Boycotting a couple of gasoline brands will bring overall gas prices down.
Status: False.
A boycott of a couple of brands of gasoline won't result in lower overall prices. Prices at all the non-boycotted outlets would rise due to the temporarily limited supply and increased demand, making the original prices look cheap by comparison. The shunned outlets could then make a killing by offering gasoline at its "normal" (i.e., pre-boycott) price or by selling off their output to the non-boycotted companies, who will need the extra supply to meet demand. The only person who really gets hurt in this proposed scheme is the service station operator, who has almost no control over the price of gasoline.
The only practical way of reducing gasoline prices is through the straightforward means of buying less gasoline, not through a simple and painless scheme of just shifting where we buy it. The inconvenience of driving less is a hardship too many people apparently aren't willing to endure, however.