However, our gating factor is time and attention - we only have a fixed amount. I suspect that what makes many HN discussion valuable is that the few top discussions draw most of the focus of a large part of the community at the same time. Without that concentration of focus, you don't get those spontaneous interactions where someone makes a comment about a decision made in a 30 year old piece of software powering half the Internet, and the guy that wrote said software responds with the rational for why he made that decision at the time.
Long lived threads and resurrected discussions from the past diffuse that time and attention. While I like the idea of longer term discussions (a lot!), spreading the beam of focus gets us less 'power on target' for the topics of the day.