Deleting an answer should check for edits, and abort if the answer has been edited since the page was loaded. If the answer has been edited, there's a significant chance that the reason for deleting the answer no longer applies.
I think this is most relevant for the moderator immediate answer delete action and convert to comment action. 20k deletion at least requires several people to see the answer, which reduces the risk of a deletion on obsolete grounds.
This isn't a theoretical concern: I'm posting this because it just happened to me. This answer was originally just a reply to a comment on the question, and (correctly) flagged as “not an answer”. A few minutes after I'd loaded the page, the author edited his post and turned it into a real answer. Then, I went and converted the answer into a comment, only to see that the answer had become a lot longer. I restored the answer, so I think there's no lasting harm done, but there should have been something to warn me I was acting on obsolete information.