there are impossibilities in life, for instance this thread can only progress not regress, that is, it can't go in the negative.
it wil never reach zero posts, mine is number 1.
Who will dispute me?

Me. While it might not happen in this thread, it is possible for the thread to have a post deleted and regress. It is also possible for every post to be deleted and the thread to contain zero posts.

Going into the negative is more difficult but I can think of at least one way that could occur on certain forum systems. Some keep a count of the number of posts, adding and subtracting as posts are added and deleted. All that might be needed on such a system is one increment to fail. If you then delete all posts and the count is reduced by one each time, you have gone into the negative.

It might also be possible, depending on the datatype, that if you kept adding posts, you could eventually find yourself presented with a negative instead of a positive number.