Nevermind that sales has nothing to do with quality or interest, either way. It doesn't matter to any fan of Warhammer Fantasy whether Age of Sigmar is doing well or not (although it's not), because ultimately, GW shat all over the fans and End Times not only attempted to ruin the setting on some kind of principle, for no sensible reason, but Age of Sigmar holds no interest to fans of Warhammer Fantasy.
End Times/Age of Sigmar share some common themes, but in terms of fluff it's completely divergent, and in terms of concept it's completely different.
Age of Sigmar appeals to the lowest common denominators of flashy "epicness", gimmicks and low attention span, with no serious depth or ambiguity to it, whereas Warhammer Fantasy generally appeals to people that would be considered more "hardcore" or "purist", and actually enjoy the idea of a universe of ambiguous power structures, unclear motivations and metaphysical aspects, and mud-covered peasantry slogging it through fields that they will promptly die horrifically in.
These two things are completely incompatible, no matter what sales figures say. Some people enjoy lobster, while others are happy with eating Big Macs for breakfast, lunch and dinner, and can't comprehend what the fuss is about; it's just some kind of crayfish anyway, right?
My gripe is ultimately that there was no financial or sensible reason to shit all over the old fans just because you decided to stop catering to them. They deliberately and knowingly went in and gutted the entirety of the old universe, with wide-scale retcons and utterly nonsensical scenarios, and completely disregarded established tropes, concepts and aspects of the setting and it's fluff.
They could've established their new universe without doing that, but for some spiteful, scornful, bitter reason, they did.