Disposable

I think they do. There's only so many times you can flex a piece of steel before it cracks, no matter how well you treat it otherwise. I have a 1984 Trek 620 that I put over 50,000 miles on over the course of 17 years. I rode it long enough to be completely confident that it was a well built frame. It was no manufacturing defect that caused the downtube to crack after 12 years, or the steer tube after 15 years (both of which Trek repaired or replaced with no hassles), or finally a rear dropout. They were just worn out under a heavy rider and a lot of cycles. I still have that bike, in a place of honor over my workbench, and I've had had other steel frames. For me they last a long time, and when they fail they do it slowly, but after a while they do fail.