An important motivation for a FOSS OS for phones is not having to buy a new phone just to have up-to-date software.
Also, "people" who buy a Google-Pixel-level phone every two years are likely among the richer... let's say 10% of the world's population? Probably even less. The rest - don't do that.
Reducing waste is very important, but I think this is something you need to take up with the Android OEMs. GrapheneOS can't really do anything about the fact that Android OEMs stop supporting the device and allow vulnerabilities to go unaddressed. For context in this situation, GrapheneOS is also trying to provide a best-in-class privacy/security experience for people. There were other projects that are/were dedicated to supporting abandoned hardware.
A connected world full of devices with excessively vulnerable hardware & software is also something GrapheneOS are desperate to avoid.
Also, "people" who buy a Google-Pixel-level phone every two years are likely among the richer... let's say 10% of the world's population? Probably even less. The rest - don't do that.