That's my plan too. I do appreciate devices are coming and will be buying them, but the whole point is to trust custody of the software, which isn't handled by the preinstalled os in any scenario with any os.
Preinstalled GrapheneOS would be funded by state Honeypot operators, or potentially thieves or blackmailers.
If you wish to steal from someone, select from a population of thieves since they won't go to the police (although they may use illegal means to punish you).
Going with the stereotype that people doing illegal things prefer a secure OS (and are not smart enough to pick a safe source).