Sandboxed Google Play doesn't really defeat the point of GrapheneOS. Not wanting to use Google Play or any proprietary Google services is perfectly valid, for all clarity. But, it's just important to note that for people that heavily use Google services, the advantage of using those on GrapheneOS instead of on a GMS-licensing OS is very high. The GMS-licensing OS will bundle the Google Mobile Services (Google suite off apps, Google Play, ...) in a privileged way. They won't be treated as regular apps, as is the case on GrapheneOS, but will have invasive access on your phone. In general, GrapheneOS' many hardening and privacy features allow you to have a better grasp on privacy-invasive apps.