I have to say, I'm GLAD Shannen left and I didn't miss Brenda one bit. Now please read to rest of my post to understand why. 90210 is definitely my favorite show of all-time and Brenda is an excellent character...
BUT, starting with season five, the show had a HUGE decline in quality and I'd hate to see them ruin Brenda like they ruined all the other characters.
Donna, once a cute and funny character turned into this unsufferable goody two shoes who everyone is in love with, then Kelly turned into this self-rightous bitch, etc. etc. Had Brenda stayed, I'm sure they'd just turn her into the Val character, thus ruining the backstory we know of her. In addition to the horrible character development, the writing was truly awful.
Shannen left at the perfect time and will forever be remembered as one of 90210's best characters and it's all because she left. Had the show ended in season four I would LOVE Donna, Kelly, Brandon, etc. but after suffering through those later years I hate all those characters.
Brenda and Shannen were very lucky!