When the time comes to replace those airframes as a general rule the new airframes are more efficient and should cover the cost of the replacement over time. Generating revenue to replace aircraft is a company function. If you take a look at the total cost of leasing and purchasing aircraft its not nearly as significant as fuel. The next generation of aircraft in the larger narrow aisle seat count may gain enough to actual be cheaper overall including the cost to purchase or lease. This will be especially true if fuel prices increase.
I am not sure what the issue is with 757 airframe life. Its the exact same tube as the 737. Same construction just longer. Fedex is buying lots of used 757's and plans on keeping them in service for 20 more years.