There are great American players who went through times not dominating. Conners and McEnroe usually did not dominate Borg, in fact there are more periods Borg was dominant than they were, when they played jointly. McEnroe only had 2 dominant years in his whole career 81 and 84, Conners only 74. Remember they both had long careers.
Agassi was never really the dominant player. In 99 he won two slams and reached one final, but had a 1-4 head to head with Sampras so there was no dominant player that year. In 95 he dominated regular tournaments yet won only 1 major to Samprass 2. So he is a true great who has never dominated a year. Sampras is the only American who dominated regularly.
If you want to see an American dominating, good luck, it wont happen. With Federer so superb, Safin coming into his own, Hewitt so determined, and youngsters coming up, Roddick will do well to stay among the main contenders. The other Americans do not have top 5 potential, and there are none on the horizon that do.