Major spoilers, this question is about the end of The Amber Spyglass. I won't put everything into a spoiler block.
At the end of The Amber Spyglass, Will and Lyra (and an angel) discuss whether there is a way for the two to stay together. They can't just stay in one world because of the deteriorating effect, they can't keep a window open because the Dust would flow out and they can't continue to make short-lived new portals because it would create Spectres.
But Will is an adult now and can see the Spectres. So he could just make a window, kill the Spectre that comes out using the knife and close it behind him, right? Or do the Spectres not appear directly at the window? If not, where do they appear? It seems like whoever makes a window doesn't see a Spectre directly flowing out of it, otherwise it would have been more obvious to the original owners that they should stop and Will would have seen one when he makes his final(?) window to show the angel how to close them.
Did they just not think about this possibility after they noticed that they can't ask the Alethiometer? Or is there something I'm overlooking (like Spectres appearing elsewhere, if yes, where)?