What enables oligopolies to earn sustained profits over long periods?

Perfectly elastic demand is characterized by consumers being highly sensitive to price changes, which is not typically the case in oligopolies. Instead, firms in an oligopoly face a downward-sloping demand curve, allowing them to set prices above marginal cost without losing all their customers. Thus, this condition does not support sustained profits for oligopolistic firms.

Allocative efficiency occurs when resources are distributed in a way that maximizes consumer satisfaction, typically seen in perfectly competitive markets. Oligopolies, however, often produce at a level where price exceeds marginal cost, leading to a deadweight loss. Consequently, allocative efficiency does not explain how oligopolies earn sustained profits.

While product differentiation can provide some market power, it is not the primary factor enabling oligopolies to earn sustained profits. Instead, it is the significant barriers to entry that primarily protect these firms from competition, allowing them to maintain higher prices and profit margins over time.