I am the one who has been sending out the individual emails quoted here as well as the email blast from Dave Simeone. Let me clarify what will happen.
1. On December 31 the technology preview will expire. It will no longer operate. You will no longer be able to use it -- not even the old version..
2. The Civil 3D team is considering extending the technology preview. If they do, this means that you would have to go to the Labs site and download a new installer for the tech preview that has a different time-bomb date baked into it. This will not happen before December 31, so there will be some days where you cannot use the technology preview at all.
3. Technology previews are temporary in nature. No one should confuse them with perpetual functionality. To provide a long term solution, it is quite possible that the team could create a version of the technology that users could download from Subscription Center instead of Labs, A version on Subscription Center would not have a time-bomb in it.
I hope this clarifies the situation, and I apologize for the confusion.
Thanks for the clarification, Scott.
I've found the River Analysis extension to be a valuable addition to my work efforts. I hope that the product will eventually be a download available to subscription customers but understand that this is a decision not yet finalized.