Back when I worked for Kodak/Sierra Nevada the Healthnet guys rode Giant TCX frames. Some of the guys definitely complained about their frames losing stiffness over the season. A guy I know that worked for Schwinn (I think) at the time took a new frame and an old frame to a facility that they had in CO and checked the frames on a rig they used. As the story goes the old frame was around 20% more flexible in certain areas compared to the new frame. These days I'd say that's not possible unless some major mistakes are made.