There appears to be a fan backlash every time some major change is made.
1. Buffy leaves high school for university and people who were comfortable with the first three seasons of Buffy get nostalgic for the core four and their tight bond.
There were enough monster of the week episodes in the first three years that a cohesive story arc only appeared in the third year. Do you really want a story about high-school students who never leave high school? People have to grow beyond that point because the story gets too limited. How many more good stories could have been told? I was one of the people who laughed at Beverly Hills 90210 because the students were in high school until their old age pension cheques came in.
2. Buffy is resurrected and her new life is a mess with the need to take care of homeowner issues, her sister, the breakup of the core four and life going off the rails. And she still has to fight demons.
There are people who would have been content to see the story end at Season 5. Buffy sacrificed herself to save the world. But in some ways, death was easier than the life she gets reconstituted into. She has never exhibited any pathology but she now has PTSD and depression and has the Trio and Spike to deal with. Welcome to the adult world, kid.
3. The potentials arrive and although they fumbled the entire Season 7 story line away for various reasons, people objected although that was more realistic - too many new characters who were whiny little b!tches (except for Kennedy, who relished the slayer task) and a cluttered story line that did not develop anything properly.
This has been hashed out in other threads where the faults of Season 7 are discussed, but the idea of a single slayer and the tremendous run of luck plus the attention of close friends that has kept Buffy alive for this long gets too incredible. Buffy herself laments that she spends her full time fighting evil, but she is making no headway in eliminating it. The only way to make a difference is if the role of slayer is opened up to other people. She then gets to plan training and provide organization for slayers with a much greater prospect of living not so much a normal life, but at least a normal lifespan.
4. Buffy leaves the crater than was Sunnydale and sets up internationally which is followed up in "Damage 5x11" and "The Girl in Question 5x20" in Angel.
This could be considered the final phase of the slayer saga: establishing a slayer presence anywhere in the world with the prospect of being able to react to any demonic presence that pops up. She has not checked in with Angel since the demise of Sunnydale, so Andrew can get away with saying Buffy doesn't trust him anymore because he is now at Wolfram & Hart when he comes to retrieve Dana. Angel and Spike both look for Buffy in TGiQ but it appears that Buffy doesn't want to be found. The only incredible thing about TGiQ is that Andrew goes from gay boy to playboy in less than a year with two girls in his room. Maybe he is the first bisexual character on TV.
If I was into the comics, I would miss Sunnydale, but we never have to go without it for more than a few minutes at the end of BtVS the TV show. I think the model for Sunnydale was possibly San Luis Obispo, a university town with a respected architectural college and a population slightly higher than Sunnydale - 45,878 versus the 38,500 of Sunnydale. It is on the coast, as Sunnydale was shown to be in "Grave 6ABB22".
But I don't think Sunnydale rises to the level of being an irreplaceable character in the story - if the story was in San Francisco and it revolved around the scenery of Alcatraz or the Golden Gate Bridge, then you could make a case that the location was an integral part of the story. But there is nothing in Sunnydale that couldn't be reproduced in any coastal college town in the US. The hellmouth could have been anywhere.
