But if the user with the higher karma is the only one interested in that thread they could easily just ignore the posts they disagree with so they never get endorsed. It leads to the same outcome, yet they appear to have done nothing wrong.

I don't really see the issue with this unless getting the last word in publicly on a discussion forum is vitally important.

The new person has posted, the higher karma one has seen it and chosen not to reply thus ending the conversation. If there is really no other attention then nothing has changed from the older person seeing the reply and ignoring it.

The entire issue is that you don't see a problem with censorship. They would have to endorse ANY comment, not just replies to their own posts.

This isn't censorship and I think that word gets thrown around much too loosely.

If there are multiple people in the discussion then it is very likely more than one will pass the threshold to vote. If there isn't then nothing of value is lost as you still get to make your comment to the other side, you still get your last word in, it is just not public and the conversation will die immediately. Much how it is now if the other side doesn't reply to you.