Or they could have gradually introduced new regulars in the earlier seasons.
Okay, technically they did by upgrading Amanda (season 2), Sydney (season 3), Kimberly, Peter and Brooke (season 4) to series regulars, but they could have added 1 or 2 more new and fresh people at the same time. All these new regulars had been on the show before in a recurring capacity.
Before season 4, the show only had 8 regulars (9 in season 3). They always had so many guest stars, so there would have been enough space to fit in some extra regulars.
BH90210 was lucky that they had 5 cast members who were always happy to renew their contracts until season 9 – and their seasons were just as long as Melrose Place's were. I think if either Laura Leighton, Courtney Thorne-Smith or Grant Show would have stayed on the show after season 5, season 6 wouldn't have been that bad.
I mean, I've always liked Jane and I was glad to have her back for the final season, but her exit in season 5 didn't really leave a big void. Jane and Sydney needed each other as characters, and they didn't really work on their own. Same with Kimberly/Michael, Amanda/Alison, Billy/Alison etc. Maybe that's what hurt the show in the long run.
Another thing that damaged the show in season 6 was the change of scenery. Familiar locations like Shooters and D&D disappeared, and too many scenes didn't place at the apartment complex anymore. The building was the heart and center of the show, but they didn't really acknowledge that anymore.
I remember this one scene in early season 6 when Coop moved in and Megan told him everything he needed to know about the other tenants. And I was like, "Okay, how long has Megan been living here? 4 weeks? And does she really know anybody of these people?" I mean, how exactly did Megan know Craig, Billy, Samantha or Kyle? As far as I remember, they never shared a meaningful scene together.