It is not just the SOF kayak market that is shrinking; rather, the entire touring kayak market has shrunk in favor of smaller kayaks. Every major manufacturer (Eddyline, Necky, Delta, Hurricane, ...) now offers a 12’ foot boat that has high degree of initial stability.
As a novice SOF builder but a long term kayaker with lots of touring experience around the world and an experienced canoe builder (about a dozen strip canoes over 40 years), i actually found the information gathering process before building a SOF kayak more complicated than it should be. Maybe this is the curse of the Internet. There is not one single definitive place for someone to go to get a comprehensive and up-to-date view of best practices for the novice builder. Personally, I started with Jeff’s book but your videos and blog posts are not always consistent with the book For example, you seem to now recommend using the double cord method of sewing the deck seam - a technique not covered in the book. But, having restored a couple of wood canvas canoes I am left wondering whether the prospective first time builder wouldn’t be better off being told to use staples and avoiding sewing altogether.
I also think the prospective builder is likely overwhelmed by the dizzying array of alternative designs available by you and others. When I am in the market for a new hard shell kayak I can go to my local dealer and try out multiple models in an afternoon before deciding what boat to buy. This option is obviously not open to the prospective builder. i do not have a good suggestion on how this problem might be solved.
If it were my business I might be tempted to update and publish your book on line with associated videos in order to drive retail sales. Just an idea.