Oh, I'm not discounting that, for the most part, there was some planning before hand, what with their writers summits and everything. But, as Farmer pointed out, plans can change. After all, it's one thing to outline a story but it's another thing to actually write it. After all, according to Steve Wacker, they knew why Harry was back but didn't yet have a working story as to how to fit this in until later. And it's pretty apparent that, while they knew One More Day was going to be controversial, Marvel grossly underestimated the intensity and length of the backlash from fans.
Also, if it took multiple writers a full year and 40 plus issues just to set things up, then something is really, really wrong, regardless of whether the stories were planned out ahead of time or not.