As stated above, as long as the airline takes the step to follow all "suggested" mx procedures and treat the a/c with respect then there should be no issues with flying such an a/c for more than 30 years. Another issue is that money + plane = Ch. 11. These airlines own the aircraft so why not invest a larger sum that will soon make itself up time and time again? It's worked for NW
and many other airlines so before you judge an a/c by it's age take a look at the facts.