Let's say I just wrote a reply A but changed my mind and would like to delete A. Assuming I'm fast enough and my reply has not been responded to - whether getting a sub-reply or receiving votes, it should be possible for me to delete A. Say while in the process of deleting A, another user composes a reply to it. When he's done and clicks Submit, A has already disappeared so his submission should fail and he should get a message saying the parent reply no longer exists.
It's a very small benefit to have, so I guess it doesn't justify the troubles to tweak it.