I can't really comment on how things are in the states as I'm over in the uk, but I also spent a few years living in Germany too so have a little perspective from there. I think the points which you make which would lead to the idea of America being in decline are echoed around Europe too. As you point out, GdP, and influence both militarily and economically are still doing very good. However, you also say that young people are working harder than before as well as being labeled (often but not always) as lazy. For me the American dream is that we can work at whatever we want, and if we work hard enough at it we will be rewarded. However more people are seeing this not to be the case, with wealth being concentrated in an ever shrinking number of accounts while I am expected to pay 10 times the price for a house than 25 years ago on a wage that when adjusted for inflation is now less, even though I'm working twice the hours. So while america the nation state might be doing very well, the dream itself is dying.
That's my little rant /observation. Feel free to correct me if you feel I've got something wrong in my Analysis.