Would you rather write a single novel of esteemed literary merit, or get filthy rich writing a brainless, fantastical series with no kind of academic recognition?
If the fantasy series was truly brainless, I would much rather write one novel of literary merit than write a series for the sake of writing a series and selling copies. Of course just because a novel has wizards, vampires, and werewolves, doesn’t mean it’s brainless.