Using Sandboxed Google Play doesn't defeat the purpose of using GrapheneOS and neither does using Google apps. It does not exist specifically to avoid Google apps or services. It exists to provide a highly private and secure OS retaining high usability and app compatibility. Being able to use sandboxed Google Play is an important part of what it provides. Many GrapheneOS users don't use it and many who do use it are only using it in a dedicated profile for a small subset of apps but that's not at all required to heavily benefit from GrapheneOS. Moving to more private apps/services over time does make sense but it isn't mandatory and users can choose what kind of compromises they wan to make.