Furst isn't specific, but considering Web of Shadows ends with Vakama telling Takanuva, Jaller, and Hahli "Now it's time for you to make new legends," I'm pretty sure 6 matoran becoming Toa for that arc was always the plan, not to trot out the Toa Nuva again. See also how Jaller gets Lhikan's Hau and even as far back as 01 was singled out for being in the clan of Lhii, whatever that was supposed to mean.
I personally feel like Metru Nui as a concept has a fair amount of merit, but I don't understand why people think staying on Mata Nui would have been bad. Was Lego Island 2 enriched by bringing in Castle and Adventurers Island? Sure, Mata Nui could have gotten boring if they kept showing us the same places and situations over and over, but they wouldn't have had to do that. Mata Nui's a big place and it's not like after 3 years we had seen literally everything on it. And I'm taking for granted here that you aren't saying we needed new characters or something, because there are endless examples of long-running stories out there that keep to a core cast of characters - that's just kind of how stories tend to work in general.
All that said, though, I'm not even that opposed to the idea of leaving Mata Nui - what bothered me more was the concurrent rapidly decreasing investment they put into keeping the story accessible and, more importantly, interesting. All the characters became flat and there was no reason to care because it was all so impersonal. But that's neither here nor there.
I'm pretty sure that's inconsistent just because Greg writes by the seat of his pants and doesn't try too hard to make sure all the details line up. I was re-reading Mystery of Metru Nui recently and Greg has Ehyre tell Nuju more or less "the Great Disk is in such a dangerous place, you're probably going to die." Come to reveal the location in the next book, and its... on the side of a Knowledge Tower. And Nuju had already climbed up a Knowledge Tower to save Ehyre, before Ehyre said the whole "ohoho its so deadly" thing. Even better, it is implied Ehyre didn't even know it was on the side of the Tower, he just though it was on the top somewhere. Greg establishes that Knowledge Towers have convenient chutes leading up to their roofs earlier on.