The purpose of GrapheneOS isn't providing a less bad operating system for insecure devices which still lacks anything close to reasonable security patches and protections. It would not be possible to provide the core GrapheneOS feature set on those devices. More importantly, they'd have many years of missing firmware, kernel, driver and HAL updates. Those are among the most important security updates and would not be available in practice. That would not be GrapheneOS and is not the purpose of GrapheneOS.