I don't care about AoS, I strongly dislike its aesthetics.
But it's cope to say it hasn't been successful. The AoS GHB was the point that kickstarted the current GW boom before 8e 40k. GW would not be making shitloads of minis and side games for a failed IP if it didn't sell.

Rather, it's more correct to say that Stormcasts failed to meet sales expectations while the other AoS armies have been popular.