Well I don't really care about the merchandising issue. The fact is Seven of Nine was a great character and one of Voyager's only real success stories. I wouldn't say she was the main reason for the quality improvement post season 3 however since the writers did seem to be getting their act together at the end of season 3.
I don't think Seven was a bad character. I just don't think it was necessary to drop Kes for her, I don't think Voyager should have become the "Seven of Nine show" it often did in later seasons, and I think Seven should have worn a real uniform or at least something less ridiculous.