Anytime Shannen leaves a show it's voted as the biggest mistake the people made, if you go to a site like Jumptheshark.com they have a poll to vote on about when exactly a show went downhill and the number one for both 'Charmed' and 'Beverly Hills 90210' is classified as "Exit Stage Left - Shannen Doherty", so for as much as some people seem to hate her, clearly she's making these shows and doing something right. They at least are appreciating her talents.
Charmed's Jump the Shark page -
http://www.jumptheshark.com/topic/charmed-general-comments/354
BH90210's -
http://www.jumptheshark.com/topic/beverly-hills-90210-general-comments/129
aside from that though, forcing Dylan and Kelly on everyone was horrible....they weren't even that great of characters if you as me, more like really annoying for the most part; kicking Brendon off as well was wrong...it was about the kids from Minnesota moving to the West Coast and they're last names were Walsh...not everything else - really people. And if they recast "Brenda" as someone said, I'd so boycott anything about that show from then on, I mean they did try and recast her in a way by bringing in the character of "Vallery" who they tried to make just like they said Brenda was in the last season, but you can't top the original and had Brenda known some of the things that girl did and tried, especially with her brother...oh yeah, that alone would have been a reason to come back and just beat her some butt; you didn't mess with Brendon after all without a visit from Brenda about it.
Like 'Charmed' there are many reasons you can say this show bombed, but to me, the main one they'll always have in common is losing a star like Shannen.