@Yodhrin, (Sorry for taking so long to reply)
I suppose AoS and WFB could be "disconnected" by soft retcons but only after about a decade of development. Alot of novels and audio books outright reference the old world so that'd make things somewhat tricky at this time or the near future.
I don't see why they should really bother, though. Just make two timelines that split off on Mannfred's decision to interfere during the climax and go from there. It's like you said, all the AoS releases can fit into the vastness of the old world. So GW could continue with it's current line and fluff while making alternate lore for WFB while FW can make models to please the old guard and specialist releases like Mordheim and Warmaster.
Your statement of "still at that stage" is a bit of a reach as it was just a flavorful boost that all factions eventually received. A WFB equivalent would be somewhere like "Nuln's supply wagons have arrived! The forces of Order get one re-roll result per unit due to the newly forged weaponry!".
Unless you mean the old world magic items which are in the generals book and Silver Tower.
Why not just have it different time periods? Half the characters in the army books were dead or from a different part in history. Just like how 40k currently has the Horus Heresy series and the traditional 40k series, Fantasy can have AoS (The 40k equivalent) and The End Times (Horus Heresy Equivilent.) Everyone wins, everyones somewhat happy.
I've said it before, but as much as the End Times had some sloppy writing I saw it bring more people back into the hobby then anything else in the past couple of years.
Agreed.
@Bartolo, they actually come up with new characters with almost every novel, with examples ranging from a Stormcast leader of the Hallowed knights who makes a arduous journey through the realm of death to treat with Nagash and meeting fun things along the way like a friendly vampire priesthood who turn into bloodthirsty monsters at the drop of a hat or several leaders of either Devoted warriors of Sigmar or just villagers who've been living in caves and hiding in realms overrun with chaos and non-human characters like a Skaven pestilence priest who completely shocks his master by showing a unheard of trait, complete loyalty.
The characters from the old world really don't feature much except to set-up what's occurring in the overall scheme of things. I believe only Mannfred is the one with the most proactive role in the novels and not running things behind the curtains.