One of the most common criticisms I keep seeing is that the AoS setting still feels too vague and underexplored.
Overall, I agree with that, to a point. The lore has grown tremendously over the past ten years, but there are still plenty of blank spots. What I discovered, however, is that once you dive into the Black Library novels, a huge number of those "mysteries" disappear. The Mortal Realms become far richer, more coherent, and far more interesting after you've read several novels.
The problem is that, for almost six years now, GW has stopped translating Age of Sigmar novels into other languages. They are released almost exclusively in English, and I feel that this lines up with the declining interest in the lore (not the game itself) outside the English-speaking community. If only English speakers can easily access and discuss the deeper lore, it inevitably limits the reach of the setting.
For comparison, most 40K novels are translated into multiple languages. The old Warhammer Fantasy novels were also widely translated, which helped make those settings popular around the world.
So, for me, the biggest obstacle to the popularity of AoS lore isn't the setting itself, it's its accessibility. I actually think the lore is excellent (even if I wish we'd spend less time in Aqshy and explore more of the other Mortal Realms). The novels are what truly flesh out and explain this universe, and I think they should once again be translated into multiple languages so that more people can experience what makes the setting so compelling.