You say that there aren't many new and exciting places that the DS can go now, and maybe you are right, but for me that isn't so bad. I would be happy for the DS to spend the next five years playing host to games as good as The Phantom Hourglass, Advance Wars and Ninja Gaiden Dragon Sword. In fact, I am really quite excited about some of the new games that are on the way, Soul Bubbles looks particularly fresh and fun.
As for the PSP I think you hit upon a good point in distinguishing between consoles that opt in to the graphical arms race and consoles that don't. Those that do are duty bound to replace themselves with flashier hardware and I think ten years is a bit of a stretch for Sony's handheld whereas the DS doesn't seem to be prone to ageing in quite the same way.
It seems that in handheld gaming, what's good for the goose is not necessarily good for the gander.
Communism means you have two cows but you share the milk with the state.
Surrealism means you have two giraffes and the state requires you to take harmonica lessons.