Why might the cartel model still make sense even when OPEC produces less than half of the world's oil?

The cartel model would still make sense even when OPEC produces less than half of the world's oil because the prices of oil would still be determined by the OPEC countries.

Explanation:

No matter whether the supply of oil from the OPEC countries reduces or increases, they would still be in the position to determine the price of oil. This would be due to the existent oligopoly which is the only one in the world as far as oil is concerned.