Yes the show went downhill same with 90210.
Shannen brings so much to the shows she appears in. Once you've seen the show at its very best with her in it its hard to accept a below par revamp.
I won't deny that Shannen is a great and passionate actress. Yet, when it comes to the quality of BH90210, I don't think that seasons 5 and 6 were worse than seasons 3 and 4 from a creative point of view. Of course the characters had grown up and storylines had become more soapy, but it was still very entertaining. What I disliked the most was that the college setting, which was so brilliantly portrayed in season 4 and influenced most of the gang's storylines, was sidelined in the later college seasons. Even the graduation episode at the end of season 7 totally fell flat. There was no emotion whatsoever.
Season 7 was when the show really fell apart, and seasons 8 to 10 more or less felt like a daytime soap that took place in a town in the Midwest and not in glamorous Beverly Hills. But like all long-running shows, BH90210 had to suffer from budget cuts, and head writers and producers changed a couple of times.
I can't speak for Charmed since I only caught a few episodes here and there. (I must admit I liked Rose McGowan, though. I couldn't stand Alyssa Milano's character.)
We don't know how the new BH90210 writers would have handled the character of Brenda in the later years. She could have become just as stereotypical as self-righteous Kelly, angelic Donna, bitchy Valerie, obnoxious Clare, goofy Steve, bland David and goody-goody Brandon. It's like they picked one of their character traits and decided to simply rely on that for the rest of the show.
If I recall, Lindsay/Janet was recurring character at first so she wasn't an obvious replacement, but yeah, she filled the "Claire" role.
Carlie played by Hilary Swank, who was a regular during the first half of season 8, basically replaced Clare. Carlie in turn was replaced by Janet, although Janet didn't have much to do at first. She only came into her own until the High School Reunion episode at the end of season 8. However, I don't think that Andrea, Clare, Carlie and Janet as characters had that much in common.