@drahardja
I've also been unpleasantly surprised by broken reply chains.

Pure speculation: I wonder if this was a deliberate design choice, where the developers were concerned about thwarting hostile edge cases.

Imagine this reply chain (A→B→C):
- A is a good post!
- B is a nasty toxic troll!
- C is a reply to the troll.

If B is deleted (either voluntarily or by moderation), if C's reply still appeared in the thread, it might seem like an unwarranted response to A (thus feeding the fire).

I don't know if my speculation is correct, and would be curious to learn the history in any case. But that's the rationale that occurred to me.