Wasn't it proven that the addition of Jeri Ryan had no effect on Voyager's ratings?
While it she didn't help massively in the ratings, I'm sure she did help in her own way. At the very least Seven probably made more magazines write more articles about Voyager. Ratings were starting to fall for everything by that era.
A very interesting graph to say the least.
The tough thing is that making a quality series does not guarantee good ratings, as evidenced by DS9's nearly equally poor ratings... As the saying goes: "There's no accounting for taste."