I just changed a rear tire. It’s center knobs were still present but noticeably worn (probably around 75% worn) and the shoulder lugs were still strong. I could have continued to use the tire for sure, but man was there a MAJOR improvement in traction in technical climbing with the new tire. Maybe that had a lot to do with riding in the loose scree here on the Front Range of CO.
I just found this thread and thought it was kind of interesting as I just replaced my rear tire. I went from a 2.10 stock tire to a 2.35 Bontrager XR3. The old tire was original to the bike and in about a year it was looking like a baldy. The new tire rubbed the frame until I took a rat tail file to the mount slots and basically did a rough line bore on them. These aluminum frames grind easy. I love the larger tire. More stable ride on loose scree, softer with the suspension cranked hard which helps on climbing, and more ground to pedal clearance. While it raised the bike almost a half inch overall, it didn’t change the seat to pedal geometry any. Rides like a new and improved bike.