I don't think the boards are dying. The activity is just in a low period at the moment. I feel Bionicle G2 helped bring in activity, but the cancelation ended up throwing everyones mood out the window.
Given that I also joined during 2014, I can say that I have a good feel for how activity works. Activity spikes when something big happens like how when G2 was announced.
Now time for an anecdote: My activity has gone down because of several factors:
I feel that making topics left and right is not a good idea. I used to make topics in the entertainment category left and right. But then I remembered TTV does not have unlimited space, so I chilled out.
My topics have poor engagement. My topics never really go anywhere and I end up getting demotivated.
But ultimately there are a lot of factors for decreased activity and it is not that easy to pin down. My reasons are G2 getting canceled and lost motivation, but those may not be the same for others.
Sorry for the tangent, I just felt like elaborating.