I don't think I ever countered a statement you made, but if popularity doesn't determine quality in a book series, then what does? Anything with the intent to sell is determined good or bad depending on how it sells, and what the consumer thinks of it.
You want a blunt answer?
Balls. Animorphs had balls. Think about it, have you ever read
any other series aimed at children that included half of this stuff? Grey-area morality, shellshock, nightmares of battle, traitors, shifting alliances, complex uneasy relationships, allusion to swearing and gore? It's absolutely unheard of.
Your statement about commerce being the defining factor in a quality product is ridiculous. Miley fuggin' Cyrus sells a bazillion records a year, does she have artistic merit?
Animorph's absolutely defining feature was that Katherine and Michael weren't afraid to go places usually deemed inappropriate. She didn't talk down to anyone, didn't pull punches, didn't pretty it all up to make it a clean "good V.S. bad" story of triumph over evil. She wrote a dirty morally-compromising war story.
And, you know what? Her personal quality of not being afraid to piss off a major quantity of her readership goes along
perfectly with that mindset. She had something to say, she had a point to make, and she MADE it, KNOWING there would be a backlash. That's admirable. It really seemed she sat there and thought "Okay, they're going to want Jake to save Tom, and Rachel and Tobias to be together, and the Visser killed. They're probably the three major payoffs the kids are wanting from this series." And she illustrated quite clearly that that type of thing
very rarely happens in real life.
She made an effort to teach pre-teen/young teen children something, and it simply went over all of your heads. It's pretty ridiculous.