I'm not talking about monetization, I'm talking about there being a cost no matter what. It can absolutely be "free" but someone's making food or using their space to host or something else.
Creating a slack instance is not building a community.
The "fee" is just a simple way to get commitment from the other side. As you can imagine, this is not a business that will allow me to quit my job.
I just don't feel that the rewards I receive from community organizing are monetary, and certainly don't need to be.
Nor do I want to be part of a community whose organizers are only motivated by money and don't even see any other form of motivation as valid. If that's how you see yourself and the community you're organizing, that colors everything you do and it makes you a bad community leader.
I basically organise the community because I enjoy reading books and sharing opinion with other professionals, that's it.
But your execution doesn't match that intention. What you have created is a business. You're talking about "commitment"--that's a business concern. "Sustainability"--also a business concern. I've had plenty of rewarding conversations about engineering books with people who I never saw again, and I've gained a lot of knowledge at book clubs that no longer exist. A lot of that knowledge came from people who only showed up once.
What you lose when you demand commitment and sustainability, i.e. a business model from your book club, is spontaneity and diversity. The kind of guy who commits himself to a book club every week for years is going to miss the life experiences that give context to his work. It's how you end up with solutions that are perfectly engineered to efficiently do the wrong things.