This is the billion dollar question, but having the right answer at the outset may not be necessary. In fact, most successful business go through multiple iterations on monetisation opportunities before they strike gold.
It is more important to build a product that solves problems that Goodreads doesn't solve right now, and find a way to acquire customers that doesn't rely on Google SEO.
This is a very valid argument. You have implicitly invested effort (in form of lists) in Goodreads, and wouldn't want to lose them. It's another reason why a challenger can't rely on book tracking as the primary feature to drive adoption.
I hear you. It's not broken. It does serve the primal use cases like shelving, reviews/ratings, meta-information pretty well. But there is also so much more to the experience of reading. Is Goodreads really the best we deserve?
It's hard because of this reason. But I'm certain alternate acquisition channels/go-to-market could exist. It's a matter of understanding the customer's journey and capturing them at a stage other than "discovery" or "search".
Lack of an evident business model could be a reason that turns off smart founders from picking this problem. However, if the product adds enough value to enough number of users, one could figure ways to monetise it.