If upvoting a post/comment (which gives the author karma points) would risk triggering its deletion (by making the author cross the karma threshold), then is that not effectively turning the karma system on its head?

Under such a system, if I think a post is valuable, I would then try to "protect" it and its author's account by downvoting it. Similarly, if I think a post is not valuable, I can help trigger its eventual deletion by upvoting it.