Graphene doesn't have the volume to get a custom flagship grade device made for them. So even if they get a device that ships with Graphene preinstalled? It's going to be a variant of another Android phone.

Which is, generally, not that good for Linux mainlining. Qualcomm SoCs are "meh" when it comes to mainline Linux support - some parts are there, but a lot of them aren't. It has been getting better for the last bit though?