Here's some of what he wrote about his decision in his book "The Next Hour":
"The other limiting factor on airframe life is fatigue. Airframes are subjected to fatigue testing on the ground during the certification process. An airframe is put into a machine and the structure is loaded and unloaded in a repetitive manner to simulate the fatigue wear that comes from flying, especially in turbulence.
This is good testing and so far it has served well. As airplanes get older, though, you have to wonder about the cumulative effects of both corrosion and fatigue. And, under the present system, the only way that we are really going to learn about airframes failing is when they actually start failing. In most cases, older airplanes will likely lose their viability because of maintenance costs and parts availability long before they reach whatever actual life limit might be out there.
In 2007 I made the decision to retire the pressurized 210, P210, that I had flown for 8,963.44 hours in 28 years, five months and 13 days. I had gotten my money’s worth out of the airplane and in my mind it was worn out. P210s have had a bad reliability record over the years and while I spent a ton of money maintaining mine, I had been having trouble keeping everything on the right side of what I consider to be perfect shape.
There was another factor in my decision to put the airplane down. Years ago, when the airplanes were first being built, I asked a Cessna engineer how many hours he thought one would last. He said the amount of testing done so far was probably equal to about 10,000 hours of flying. My airplane was getting close to that.
Some people thought I was nuts to scrap an airworthy airplane but I was and still am comfortable with the decision. I had gone a lot of places and done a lot of things with the airplane and knew it as well as a pilot could ever know an airplane. And when I felt like the airplane whispered “enough” into my ear, I thought I should listen.
A lot of mechanically-related problems come after pilots don’t listen when an airplane suggests it is not a good idea to go flying. If everything isn’t working perfectly before takeoff, then it’s time to taxi back and seek help. Even after taking that precaution, there will always be times when something will malfunction after the airplane is airborne. How a pilot deals with the airborne problem has everything to do with the happiness of the ending."