Let me add that the typical GrapheneOS user will probably prefer to install the OS themselves rather than trust what comes preinstalled.
How do you know this? Is there an official (or even unofficial) source of GOS preinstalled devices that a substantial amount of "typical GOS user" has acquired?
Or maybe you are talking about "potential user of GOS"?
In any case: if you installed it yourself you mostly have to trust the source of the installer. If you purchase a pre-installed device you're basically back to the android/ios model: you have to trust the manufacturer AND the maker of the OS