Deep Space Nine spent most of its lifetime as the number one syndicated first-run show on television despite its falling number of viewers. Even when it became a near-serial show (usually, long-term serial shows are ratings disasters -- witness Babylon 5) airing in prime-time in less than 60 percent of the nation, DS9 managed well over a 4.0 average in its final two years. As a general rule, a syndicated show needs to maintain a 3.0 to be successful, DS9 always maintained that despite the strikes against it. Look at the other sci-fi shows similar to DS9: Earth: Final Conflict is regarded as a decent show ratings-wise, staying in the lower 3.0 range and Babylon 5 is the hot potato of science fiction television -- it's done so poorly that no one wants to hold on to it.
As a serial, more cultish television show, DS9 is right behind the X-Files on the all-time list of successes even with extreme disadvantages.
Voyager, on the other hand, has very little that it can brag about. That's not because Voyager is an awful, unpopular show, but because it's on an awful, unpopular network. Voyager can only do as well as UPN because of Voyager's status as a network show. UPN has been losing stations since day one and is now only airing in a little over 60 percent of the nation, meaning that Voyager is competing in a very crowded market with both hands tied behind its back. For its disadvantages, Voyager has still managed to remain UPN's top show. However, Voyager will never be able to perform near the level of its predecessors so long as it drags the carcass of UPN wherever it goes. That's not the sign of viewers losing interest in Trek, it's the sign that viewers aren't interested in UPN.