The question certainly does have a definitive answer because Joss Whedon himself has stated, without a doubt, that Buffy had already fulfilled her place in the slayer line by activating Kendra. At the time of her death in The Gift Faith was the active slayer and so only her death was able to activate a new one.
I haven't got a link to a source handy for this but it is a fact and it's fairly common knowledge around here and if you look around in numerous other threads on this topic you will no doubt see what i'm talking about. Either that of ask nerd4hire because i'm sure he has the relevant interview or article.