The show went downhill with season 5, when it became a family flick (when all the fairytales character/story and the first baby Haliwell were introduced in the show) but before that the show kept a great consistency and the true essence of the show during 4 seasons (dark atmosphere, blood, charm, female sexuality, feminism)
The show was still good but it wasn't the same after season 4
Weirdly what saved the show was season 5, the show got almost cancelled at the end of season 4, the ratings were good but not enough for WB who wanted to introduce new shows
I don't know if y'all noticed but the final episode of season 4 feels almost like a series finale
The WB asked the team behind Charmed for more "loners" episode, a less serialized season so people could catch an episode during the season without being lost
The writers presented a story arc that could attract a new family audience (with tons of "one-shot" episode) and WB loved it so much they renewed the show for a season 5 and 6
The show was moved from Thursday (a teen slot) to Sunday (a family slot)
That's when the show overflowed with family-friendly magic stories (fairytales, super-hero, magic school, the themes of being a mother/raising a baby were introduced)
The season slayed in ratings, the most watched season since season 2.