I've owned 3 PUs and 2 TTs in the past 25 years. I've never owned any one of them for more than 8 years. As the kids grew our needs changed. When we got older we wanted more space, more conveniences, and less hassle.
We bought all 5 new, and when we sold them, the common comment we received was "this thing looks brand new". That's because I am a stickler for upkeep and cleanliness. After looking at several used TTs on a dealer's lot, I quickly understood that many people do not maintain their units.
So the bottom line is that we didn't trade up because of the age, and we never reached the 10-year rule.
I do own a truck that is now 13 years old. While little things have broken, which is annoying, we still tow our TT with it. I hope to get 20 years out of it!