Ultimately, I bet TDH will outlive vBulletin, which is simply not architected to handle newer social networking features.
I think that's the key...
I do believe that web boards are going to fade. When I first started using the Internet heavily, Usenet was THE place to go for chatting.
I remember being on alt.gothic when people were planning "Convergence", a meetup of goths all over the world.
There were also text-only email lists, run by listserv/majordomo type applications. You could subscribe to a list and would get the emails. Some lists had an archive feature where you could go online and get the older posts in batches. I actually remember writing some code to download the archives, convert them to HTML, then run a rudimentary search engine against them, in an effort to make a searchable history of a list.
Then the fancier apps (mailman, etc...) started to have a web based archive built in.
Yahoo Groups were all the rage for a while... You could read it through the web or email, you could search it, etc...
Livejournal and "web logs" became pretty hot too, with web logs evolving into "blogs" as we now know them.
Somewhere in that timeline forums came along. Right around when I was joining the 501st the push started to transition from Yahoo Groups to phpBB. I helped move the NEG and the whole Legion over to phpBB, and that's when I really came into the joy of the web board.
But web boards alone aren't great for what we do. Enter the wiki... a collaborative content management system. Now we can build a database of all the research that's been done, that you can read like a book.
So it makes sense that the web forum isn't the ultimate form of communication.
I think that content syndication and aggregation are going to be the key concepts of the next transition. Get only the content you want from the people you want to get it from, and not see anything else.
The mechanics of that are still to be determined, but I'm sure it will happen.
I think that TDH as a concept can evolve with the trends. The community will change and adapt to new trends, and 3-4 years from now the TDH that we currently know will be something that the old folks talk about, and occasionally remind the new folks about as we walk barefoot up hill both ways in the snow to get home.