Is not a monopoly though, yes there are cartels but it’s not a monopoly. The less demand there is for oil the less power OPEC has to manipulate prices. They will be left scrambling to supply the remaining demand. Supply and demand isn’t a myth, OPEC simply manipulate supply to influence the price.
Demand does change the price. You can see the price drop during economic downturns. Supply and demand are two sides of a single equation. It simply doesn’t make sense to say that only one impacts prices.
Prices had been dropping recently despite KSA reducing supply, until the attacks in Israel that is.