You are totally correct (and yay KSP!) But the danger is the debris that is ejected from a collision at a lower orbit would then collide with debris that is at

You are totally correct (and yay KSP!) But the danger is the debris that is ejected from a collision at a lower orbit would then collide with debris that is at a higher orbit. The debris from the 2nd collision would have its original periapsis, which may be high enough to stick around for decades.