@UncleMike: There are two reasons why we do this, but that doesn't mean either of them are valid reasons for our decision... happy to discuss.
First, as you said, authors don't follow or reply to discussions on older videos, so we felt that users were mostly posting comments into a void.
Second, before we had this in place, the vast majority of comments on older videos were spam. The % was so high of all comments made on old videos, that we felt just turning them off was the right path.
I'd hate to think we were stifling discussion though, any chance you could give an example of a video, and the discussion you wanted to start? Might help to convince us that the risk of more spam is outweighed by the content that would be added to the site's discussions.