I legitimately cant see GW fusing all the Clans back together when they can make more money with less effort by just selling 4 clans that can ally.
I mean they have to make 18 units alone in plastic afaik. I dont think GW have ever done that much in a single release post WFB frankly. Not even Stormcast.
By keeping them separate its much easier. You fold Clanrats, Stormvermin and Grey Seers into every faction since every clan has expendable chumps and their slightly better bodyguards , using Skyre as an example they'd get a plastic Dual Acolyte kit, Endless Spells, plastic Weapon Team Kit, scenery item (since thats a thing people forget) and a plastic kit that can make either a Warlock Engineer or arch Warlorck.
5 kits are an easier sell then 18.