Why would they do maps of the realms that would just take us back to the Old World. Board strokes with detail added in places like 40K is the ticket. It's not an RPG setting we just need to know what the battlefields are like or could be like. Sure more detail wil be added but it being a massive largely undefined setting is deliberate. Its so one can actually forge a narative that has consequence. Sure Sigmaron is a fixed point but everything else is largely up for grabs. Just like 40K there's a number of defined points but the vast majority of the galaxy is a blank canvas for the players to fill.
Knowledge is knowing a tomato is a fruit
Wisdom is knowing not to put it in a fruit salad.
They don't have to be mutually exclusive. They could create a general map of the overall area (of space, I imagine), and general concept maps of the realms, while leaving plenty of area open for interpretation.
It would also help to explain whether the realms really are floating chunks of rock in space. Right now the maps shown off so far make me think of the Warhammer equivalent of Outland, with the World-That-Was taking the place of Nagrand (complete with portals to get on and off the rocks). If that's what they want them to be, okay, but that would also limit the amount of space available.
It's also worth remembering that in some of the fluff they seem to mention worlds that sound like they aren't actually part of the realms, so in theory there could be other worlds out there connected via Stargate... er, Dark Portal... shoot, I mean Realmgate. And there you have infinite space to do whatever you want, no matter how much they define maps of the realms themselves.
- - - Updated - - -
We skipped that point with the first book, where they said, "Thousands of years of history happened, we don't care about that, let's skip to this: Chaos won, and beat everyone. But rather than destroy everything again like they just did, they bickered and fought among themselves, as they always do, except that time that we just described a few months ago. Sigmar knew they wouldn't ultimately destroy everything, even though he'd already seen them do just that in the past, so he bided his time and built an army. Now, with all meaningful civilization wiped out and no nations to ever care about, Sigmar's fighting back, and driving Chaos back." All stakes removed, just like that.
I think they're trying to retcon at least the part where all civilization was wiped out, but at this point, we already know Chaos can't win and it's just all up from here. Trying to go in media res actually ended up messing up the stakes of the story and leaving it so there realistically aren't any. They can't even fix that without a retcon of the story to date... though 40K showed they're okay with doing serious retcon work when needed (or even when not... looking at you, Necrons).