Tng was successful because of little to no competition and good writing. DS9 was successful because of good writing. Voyager and ENT suffered because of bad writting.What ever is done with Star Trek, the idea must be fresh. TNG was successful because it was new. DS9 because it was different. VOY and ENT failed because they were too similar to what we'd seen before.
TNG was financially successful because it aired before the proliferation of cable channels started that has bled off the audience for all network programming. DS9 was hit by the start of that trend, which continued with VOY and finally finished off ENT. The fact that TNG was soothing and easy to take, while DS9 was challenging and off-putting may have also contributed, but it doesn't tell the whole story.
While I agree that DS9's writing was better than VOY or ENT, (or TNG - soothing and easy to take aren't qualities I value in writing) if you actually look at the Nielsens charts, you won't see a correlation between quality and popularity.
To understand why Star Trek took a nose dive overall during that time period, you have to look at the larger entertainment trends, which are crucial because those trends have only gotten worse over time. To survive on TV today, Star Trek has to deal with the same - or worse - environment that killed ENT.