At last, some good debate :)
You're right that long-form as a means of communicating complex, impactful ideas won't go away. However, you are assuming that the average audience tends toward the ideal (being on HN for a while certainly influences that assumption).
In my short experience, I found that most people outside HN can easier express their thoughts and ideas on the spot, and will rarely take the time needed to craft a thoughtful long-form missive. This doesn't necessarily mean that their ideas are not worth our time. It does, however, point toward a less than ideal conversation partner and to future communication issues.
Although we'd all love to live in a world where communication is held to the highest standards, we have to adapt to what's available (within reason), even if that brings us down to bullet speak, Top 10 lists and click-bait titles. How else will we find a broader audience for our messages?
Your last point on lasting impact is, however, valid. I can't remember the last time I've re-read a tweet, but I constantly refer to old emails for clarification.