What I am saying with Dunst & Maguire et al, isnt that they are not replaceable, its that if Raimi only wants to work with them, or just for consistancy, it makes sense to wrap up storylines which involved those particular actors.
Also, its not about "if Raimi wants to do 6, let him do 6", its about what Sony and, to a lesser extent, Marvel want.
Rumours suggest that Raimi is trying to adapt the Hobbit, which would put a crimp in both his plans, and Sonys plans if they decide to continue the Spidey franchise (dont forget it was Superman that got in the way of Singer completing the X trilogy).
Raimi knew that despite what he wants to do, he might not be able to do that.
Sony could go ahead and make Spiderman 4-100 if they want, but only they can decided whether they sign Raimi to do those pictures.
Raimi could have planned out a shit load of things for Spidey 4, but he knew that was not a guarantee.
I think in this particular case, the fact Raimi is a comic book geek has gone against him.
Where as a normal director works on a single idea for a superhero movie, I think Raimi wanted to fit in as many bits of Spidey history as possible, and forgot when to stop.
