Old miniatures just need some pages at the end of the GHB to still be legal. That's the "magic" of AoS, you don't need to remove anything, just sculpt new miniatures and make better rules for them. People that have old miniatures will still have the oportunity to play with them, but AoS will give a lot of reasons to buy more miniatures (that's their thing).
At the same time, Cities seems to be the most fleshed out faction to me: Soulbound is 100% based around cities and how they work, 85% of BL new books have cities being the main place where things happen and the new battletome is just awesome... I mean, if they want to get rid of Cities, they are doing a really poor job.
Imho, Cities will be part of the future of AoS. With the option to soup some micro-armies in one battletome and giving us a new reason to buy new material (plastic or new books) is awesome. I really hope to see new Cities like the Floating City of Baatar or Lumnos!!