I did enjoy Kes greatly but Seven of Nine was needed. Seven brought the show more Borg and with that came darker episodes and a deeper feeling about the show in general. Like Data in TNG, She needed to work towards humanity, in her case however, it was regaining her humanity and at first it was a reluctant pursuit. With her annd the EMH, the show thrived and allowed itself to stand out as a different show than the others. Prior to seven, aside from a few epsiodes the show was warm and light hearted so it needed a good injection of the darker elements.