Monopolies can extract massive wealth from a market, they perfectly optimize profit as long as they are not exceedingly incompetent. Price fixing is just a less organized monopoly.
The problem is that they do this at the cost of all other market participants. But markets can't correct for powerful enough monopolies or cartels. Only outside intervention (riots, government intervention, disruption of the whole sector) can dissolve a monopoly. There is no example in history of a monopoly losing its position in a market without this, since they can always just buy out incumbent competitors.
Price fixing in the market doesnt work, because rival landlords cant call the government to airstrike their competitors when they get undercut.
And again, we don't have to guess. Cartels need to be broken up from the outside, they just don't dissolve naturally. This is basic economics, and a well understood weakness of markets. The idea that monopolies and cartels are too weak to resist in a market is not born out by either economic theory nor history.
These two are completely contradictory statements. "Regular market actors" are the same as "defectors." Fracking companies, for example would not be able to exist without OPEC driving up the price of oil because it would be too expensive.
iraq invasion has nothing to do with opec. And no opec country has made military moves to enforce the cartel.
And in fact, a lot of opec countries tries to skirt the quotas for personal gain!