I agree with you to the extent that the story in the last three expansions, BFA, Shadowlands and Dragonflight, was quite poor. WoD also had its issues.
However, the story in WoW was never really geared towards adults, as you put it. 
It might have been more solid back then, yes, but WoW never had truly exceptional storytelling. The cartoonish look also never gave the impression of a game designed for adults. I mean, with things like Vulpera looking like they’re specifically designed for children, everything took on an even cuter vibe, let’s call it that. But yeah, the game was never particularly mature.^^"