The model for Android is that the OS is provided by the OEM. They work in partnership with Google. GrapheneOS tries to work against this model by not partnering with the OEM on supporting their hardware with its OS. Ultimately that's not sustainable given the dynamics of the ecosystem. This is why they are moving to a model where they do work with an OEM directly. These sorts of problems will subside once that happens.

You could argue that the current ecosystem dynamic is not great for the longer term, but it will require a change from the OEMs to enable something different as they are the ones actually in control of the situation.