Although I admit that it's unlikely they will sue you for a product that isn't just a database, you shouldn't build your product on the (perceived) goodwill of the MongoDB developers.
Meanwhile, half the world start-ups base their entire tech stack on the goodwill of amazon and google, not to pull the rug from under them in hundreds of legal ways.
Using your definition of "goodwill" means basically everything on the Internet is based on the goodwill of some other entity. I think that paying for a service pretty clearly moves it out of the "goodwill" category.