Actually, bona fide examples of true predation are rare. The most famous non-case of predation involved the Standard Oil Company in the 19th century oil industry. Predation is not impossible, but it is extremely rare. We do not need laws against business behavior that almost never occurs and would be beneficial for consumers even if it did occur. Predatory practices allow firms to monopolize markets.
Predatory pricing is distinguishable from competitive pricing.
And since both short and long run marginal costs are almost impossible to measure accurately, such a legal standard would be completely unworkable. From a consumer perspective, all of this cost discussion is irrelevant anyway. If a dominant firm lowers prices to eliminate competitors but consumers ignore the lower prices and support the new firm, then the price reductions cannot be predatory. Thus, it is consumers, not dominant suppliers, who ultimately eliminate rivals and determine the market structure of competition. We do not need antitrust regulation to second-guess the decisions of consumers.