I don't think the butted tubes are less durable: my straight gauge early 90's Merlin spontaneously cracked in two spots in 1996, I left it out with the trash (I was not the original owner, no warranty, the bike didn't own me anything, I paid $1,000 for the frame slightly used and rode it 40K miles). My Ibis Ti Road with the butted tubes now certainly has more miles on it than that and rides like it's brand new. From my discussions with dealers and frame builders, Ti frame failure is extremely rare, but so are Ti frames with 40K+ miles (most cyclists don't accumulate that kind of wear on their bicycles). So you keep riding it, and if it breaks, get another bike. I'll be surprised if my Ibis breaks, and by the time they bury me it will probably have over 100K miles on it.