Why is leaving a defunct upper stage in a marginally stable Low Earth Orbit (LEO) considered a significant operational risk?

Answer

It acts as a high-velocity projectile for a decade, increasing the risk budget for active satellites.

Even if a defunct stage in LEO takes ten years to decay naturally, that time frame represents a significant operational window during which it poses a constant, unnecessary collision risk to other assets operating in that space.

Why is leaving a defunct upper stage in a marginally stable Low Earth Orbit (LEO) considered a significant operational risk?

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