As there’s no deleted topics list as it happens with 2nd and so forth messages in the topic, it’s very important to have at least confirmation box before making this action as there’s no return point.
Remember that only staff can delete topics, however.
Not opposed to a warning when the topic has (n) posts but it might be annoying to staff who know what they’re doing.
The bigger issue is that there is no undo, if you mistakenly delete the first post you are hosed.
This is improved now, Staff can get to a list of deleted posts from the user page. Of course it would have to be the user page of the user who created the topic…