Threads aren't linear - people don't read them from top to bottom like documents. Every subthread is a separate conversation which the reader may or may not be interested in, and having distinct pages makes discovery easier. Ask yourself why every other list page on the site is paginated? Why have only thirty stories listed on the front page? Why not just list every story ever posted, or every story this year, or the first thousand or hundred at a time?
And as far as "one can simply stop reading," ... yes... but this is a forum. It's primary purpose is to be read, not to demonstrate how quickly Arc can render HTML and send it up the pipe. That isn't even impressive. What is the value added by rendering a thread in a single page? How does that make the site better than pagination? What is the user getting in return for the loss in readability?