since it's presumably solid-propellant, the casing is very sturdily constructed; it might hold together until a fairly low altitude. the air doesn't get really dense until rather late. plus, what would a stage burning up on reentry look like? it might be expected to produce a very bright, and fairly long-lived, flash at high altitude - rather like what we see. by the way, even if an object burns up on reentry, that doesn't mean it just vanishes into thin air. a good deal of debris might still reach the surface.
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