Many interesting thoughts in this thread, just to share a little more of my perspective and experience...
I really don't think hunting pressure has had any impact on the decline in my area. Why? I think the grand total of birds killed in the fall in my neighborhood over at least 15 years is fewer than 5, and I can only specifically remember about 3. Meanwhile, I have found dozens of carcasses over the years and neighbors have had similar findings...these were all adult birds. Poult survival can certainly be at play too...but by no means is it the only factor present in my area.
Something is killing adult birds...although admittedly, we don't find nearly as many nowadays as a few years back. I suspect that though is related to the fact that there just aren't as many birds around to then find them dead nowadays. I used to see and hear dozens of birds on almost any fall hunt, this year...I think I saw a few turkeys one time.