Cessna publishes Service Bulletins all the time, many of them dealing with issues related to aging airplanes. Fatigue cracks are not unusual, and when Cessna sends out SBs we get serious about them and have a look, and sure enough, we might find exactly what they're talking about. It's no different from old cars with many miles on them. The 172 tends to crack at the bottom of the aft doorposts, where the post meets the gear box. The horizontal stab's forward spar cracks, as we've seen. Some models would crack the forward doorposts at the door lower hinge. The engine mount attach points inside the firewall sometimes crack inside the channel, where it's hard to see. A borescope catches that. The engine mounts of some would crack at the lower transverse tube welds. The mufflers and exhaust risers crack, but that's no different than many other airplanes. The carb air box falls apart. The flap cove skins and their stabilizing brackets in the wings will crack. The flap lower skins crack at the the trailing edges, through the rivet holes. The flap rollers cut the flap support arms and can cause failures. See, there's plenty of information for the serious maintainer. I would not be at all surprised to find fatigue cracks in airplanes maintained by people who don't pay attention to service bulletins. As far as the 150's lower rudder bearing: there's an SB AND an AD on that regarding the rudder stops. ADs are too often overlooked.