He is getting up there in age and yes he is capable of upsets. I watched his win against Roddick at Wimbledon. Roddick wasn't playing his best, they were on a secondary court, and Lu was playing out of his mind because it was his favorite tournament, the one his Dad always wanted him play but never got a chance to see because he had passed away.
As for his ranking, my reasoning is that he doesn't have any weapons other than his speed. Once you get past age 30, speed is hard to come by. Even the mighty Ferrer's body is starting to break down due to age. It's difficult to dispute that. The game has so much pounding and Lu really loves the hard courts.
I think anytime you have that kind of a background where you don't get all the resources you need (Lu never did) and you have to make it on your own, top 50 is pretty darn good. I agree he is one of the better players not to have an ATP title (non-challenger). It just goes to show tough the tour is.