I think part of it is if a large percentage of the models can be used in Age of Sigmar then no army at the moment for the Old World.  Skaven still use the same minis from Warhammer Fantasy in Age of Sigmar.  Lizardmen got a range refresh but my understanding is, unlike cities of Sigmar, they are like for like (skink for skink).  I presume Chaos in Age of Sigmar has older models kicking around in it as well.  It is also my understanding that a lot of minis outside the starter sets  for The Old World are going to be Games Workshop direct only.

I think every year we’ll see a book or two of extra armies from the dead list.  Firstly to show the game’s not dead.  Secondly as minis are shunted out of Age of  Sigmar and into GW’s recycle bin.  Thirdly need to sell more stuff.

So next Christmas expect Rumblings Up North as Chaos, Chaos Dwarves and the Norse appear and Kislev rides out or AOS Skaven evolve and the old models are flushed down the sewers into the Old World for Trouble Down Below as the ratmen appear, the dead walk  and the Border Princes start fielding Dogs of War.  The 3rd expansion might be Far Expeditions with the Norse upsetting Cathy and the Lizardmen.

@orinoco I believe this is the case.  GW don’t want people re-using miniatures between The Old World and Age of Sigmar, therefore any army which has a lot of miniatures still in use for Age of Sigmar will not be included in The Old World.  If The Old world goes on to be a success, I would be surprised if we didn’t see Chaos Demons, Skaven and Vampire Counts at some point – especially the first two as these were a fundamental part of the fabric of the Warhammer Fantasy story.

I know that it potentially hurts players of Dark Elves and Lizardmen personally, I quite like the cut down focus of the game, looking only at the Old World rather than trying to expand the view to include the New World.  I have mixed views on the likes of Cathay, on the one hand I would love to see what they do with them but on the other, much like the Lizardmen, it starts to get a bit harder to explain how/why the armies of Cathay might ever have made contact with the Empire and Bretonnia in such a way that they were fighting in each others’ territory.  There hasn’t really been anything in the lore to date that would adequately explain how that could happen and why it would.

The Lizardmen are a bit like the Snake People from Frostgrave Ghost Archipelago – they are part of  a spin off to frostgrave set in the same world as but separate to the main Frostgrave game.  This sets a smaller window on the world in which to play the game.  So perhaps a “Warhammer: The New World” might have explorers from Cathay and the Empire fighting Lizardmen and Dark Elves and even High Elves but I don’t think that’s on its way any time soon (as cool as it would be);  it probably wouldn’t expand Cathay all that much either as it would focus on their expeditions rather than their homelands.  Or you could do some kind of Eastern based expansion that had Ogre Kingdoms and Cathay (which seem like the most natural opponents) but it would start to become more difficult to include the Old World factions

This is a GW game. They will spam factions over the coming years. Day 1 is always going to be limited because they don’t have the models to sell you.

The problem they will have, for the armies that can also be used in Aos is literally tracking sales and working out profit margins. It will be as simple and annoying as that.

Given one of the starters is Tomb Kings, all bets on the “smaller window” are off since that faction is far from the “main action” in that setting.