As he pointed out: I knew about stress fractures in airplanes, and knew that the body of an airplane won't last forever, but I never thought about water pressure and stress factors that weaken a submersible over time. It comes down to unavoidable eventual failure, and while carbon fiber has been around for years, we still don't know everything about it.
I talked to a boat builder in Maine 50 years ago, and mentioned that fiberglass hulls would surely outlast wood, as fiberglass was indestructible (barring a collision with a rock or something like that). He built hulls out of wood, so I disregarded him when he replied, "We don't know that yet." He wasn't defensive, he was just stating a seldom considered fact. Low and behold, 20 years later he was proven right when we learned that over time, fiberglass in water will blister, and the blisters grow and grow like hull rot. They can be repaired of course but only if you catch them soon enough, and repair costs are phenomenally high.
Carbon fiber is at a point now where fiber glass was back then. We don't know enough about it to know how it's going to stand up in extreme conditions.