"Almost the entrire structure of the LM5 core stage is made of Al alloy, which quickly burns up during re-entry. A long piece of pipe inside it would then be exposed to the fury of the re-entry as other pieces of debris."It burns up "quickly"? How do you know this? And how quickly does it burn up? How do you know this long pipe would be exposed to the "fury of the re-entry"? And what do you mean by "practically no damage"??? I don't know about you but I'm pretty sure the pipe wasn't made to look crooked and burnt like that. To allay your suspicions all you really had to do was google "reentry debris" and take a look for yourself:
