I'm pretty sure he's taking the opposite view, that (a) building elements have indefinite design lives but don't last that long, and (b) Even stonework doesn't

I'm pretty sure he's taking the opposite view, that (a) building elements have indefinite design lives but don't last that long, and (b) Even stonework doesn't endure indefinitely. I'm not sure I really agree with the second point in this context - stonework tends to endure. We have Norman cathedrals that are looking good at nearly a thousand years old, comparable to the timescales under discussion to spaceflight. More to the point, there's no reason to believe that the basic structural elements of, say, a hollowed nickel-iron asteroid wouldn't be durable enough for spaceflight.