The only reason you see some old 747 classics still flying at $147 oil is that there are no other newer freighters to take their place. Also, as the price of oil goes up, the price per hour that the ACMI operators can get for the classics goes down because the customer pays for the fuel. I think that a classic can only garner about $4k per flying hour. After paying for maintenance, insurance, any lease costs, and the crew, the profit margin must be razor thin.
For the ones remaining, you usually see them get the bullet when they come up for expensive heavy checks.