I'd totally disagree with this point. I think the whole thing wit Rorschach journal could easily open things for a sequel of sorts!
Are you saying that was the ending's intent or that-that's what could be intuited?
If it's the former, I disagree. I saw the ending as the superlative ending. It's basically Rorschach's revenge. He pissed in Veidt and Manhattan's coffee; the damage is done; we know there's gonna be a shit storm; the implied effect of the journal carries more literary weight than a full blown sequel that'll just feel tacked on.
My personal feeling is that Moore, despite the shit he will often spout, wrote a way for there to be a sequel, and I think that it hinges on the journal.
Its well known that "decent" writers will often write a get out clause in a lot of their stories in advance, for the potential of either a return of a deceased character or for sequel potential. Marv Wolfman for instance, has said for years that he specifically wrote something in Crisis on infinite Earths that meant Barry Allen could return.
He has never stated what that get out of jail clause was, but he has pretty much said this since the start.
Moore being Moore would never admit such a thing, as planning a possible sequel would make him a "corporate whore" or some kinda shit like that.
I can almost guarantee that had there not been issues between him and DC, Moore would certainly have been asked to do a sequel, and he would more than likely have agreed!
