While not very technical, that article does bring up some good points. Do airplanes have a maximum length of time that they are safe to operate? The true answer is that no one knows. It is not an exact science. Under some conditions airplanes can be maintained as airworthy for decades and decades, while other times they aren't. It depends a lot on conditions and no one knows what the right answer is.
Some people on A.net believe that planes will last for as long as they are commercially viable and cite things like Northwest DC9s being a reason. But one event can have major consequences. The Aloha disaster is the biggest example of a plane that failed because it fatigued naturally. There are other cases where fatigue has led to disasters such as the JAL 747, but that was due to poor repair techniques.
All in all, I'm glad research is going in to this very important topic before another disaster happens. It is good to see the government being proactive. I know that article took the airline's side of view saying that it is too expensive, but whatever that can be done to ensure more safety is a good thing. I don't think people that don't know what they are talking about should make arbitrary limits. The FAA should work closely with manufacturers and operators to decide what is best. Hopefully they do.
If you have never designed an airplane part before, let the real designers do the work!