mack - 12:36 am on May 26, 2010 (gmt 0)
everything within a thread is based on context, each post sets the context for the next message. If you where allowed an indefinate time frame to make edits you're not only editing your content, but the entire thread.
as an example you post "i love foo" reply says "so do I" you then go back and edit love to hate. It just can't work.