ALL the GW fantasy computer games (Blood Bowl, Man O War, Mordheim, Vermintide, Total War) are based in the Old World, not the Age of Sigmar.
Of those, fully FOUR of them have been announced AFTER Age of Sigmar. The Old World is too valuable an IP to throw away so I wouldn't be too worred about Sigmarines. All the people our age who are the core market for that world have been catered for in that respect and I wouldn't put it past them to do that model wise if they were really bringing back such kinds of games.
I think it makes sense to keep The Old World around just in the same way that there is now Warhammer 30k and 40k. 30k had been a specialist boutique thing for years but now with Betrayal at Calth it will actually be front and centre in plastic, two eras for the same "universe".
Why not the same for Fantasy? Sigmarines and super simple rules for the kiddies and casual players, Old World for the grognards with disposable income but more specialised tastes?