There are other phone companies.

Graphene is working with a manufacturer to build a device for them.

Also, Graphene isn't the only game in town. Lineage is fine, yes, you can't re-lock the bootloader on most devices, but frankly that risk is overstated for the average person. For the people who need that kind of security, if they're that big of a target then the actors after them will have state-level capability. So it's a deterrent for less-capable adversaries.