The selling point for this theoretical device is that, even if someone steals your passcode, it can’t be hacked. This enabled evil maid (or customs processors, nation-state agencies, etc) attacks on the most vulnerable users looking to have defense-in-depth.

To avoid this it seems you’d either be forced to sell it at an insane markup (to avoid having too many users) or means test people to make sure they don’t purchase it if they’re not super vulnerable to attacks that matter.

Or just have a way that prevents those kind of attacks. Like having the trusted bootloader light a LED or show a hash code of the configuration on screen.

What are the chances some third-party OS vendor complains that they have to work with iBoot to launch their OS and demand they be able to replace it?

I think there are easy to think of solutions to the problem, that let's everyone win.

The obvious solution, for your hypothetical, would be to give a setting to the user, that turns on "safe mode" and make this setting only changeable on factory reset.

So any user could choose, at startup, to have safe mode on or off, on their own device.

Problem solved.

You wouldn't need to do anything to prevent people from buying it.

The restrictions inherent in the device (including only having a few apps that have been thoroughly reviewed by security researchers) would keep most people away.

Easy, do it like android. In (some) android devices you can unlock the bootloader. But doing so will inevitably wipe out the device.