(Luckily this series isn't based off books and has no logical reason for doing this)
Well... it's based off
comic books

t: I get what you mean though.
I'm with Eddie Brock -- it's just more prudent to plan for a trilogy, as much as I'd love a fourth movie.
Peter's ready to break his promise to leave Gwen
out, but what he's also doing is letting Gwen
in just to protect her.
I'm going to digress for a second, but this is really to reflect on what the dynamic might become for the second movie.
This is what I think is what Peter probably misses in the grand scheme of the "great power, great responsibility" lesson, and what he's going to learn in the sequel. He's not misusing his powers or anything like that anymore, but ultimately, his inherent sense of confidence is going to kill Gwen. Peter's confident that he can still be Spider-Man and still bond with Gwen, with her knowing his identity and becoming a liability.
I'm going to enter some different territory to keep going with this point. Both Spider-Man 2 and The Dark Knight explored this little motif of heroes with desires that need to be given up. And you can argue that this motif can be present at the end of the arc, but the thing is, Spider-Man can't complete his arc if he makes one of the biggest mistakes of his life. You don't end with that in a third movie and not even consider a fourth movie.
A movie following the death of Gwen Stacy has to be the movie where Spider-Man puts everything he's learned gets challenged, and the one where everything he's learned gets put into full use.
So in short... Gwen Stacy should die
before the last movie. I'm being very specific in saying that it might not even be the second movie. Because we really don't know
for sure what they might have cooking for us.