The RPi was ready to roll out of the gate - they had a working Linux distro from the get-go that actually worked. They also provided really simple instructions on how to get said Linux distro onto a SD card. You can boot the RPi and have it be useful within a matter of minutes.
Openmoko simply never got near enough to working where the community felt like picking it up and running with it.
I don't think Openmoko was premature - it was incomplete. You're not going to get much participation from the community if doing anything useful on your platform first involves fixing/finishing the platform.
Broadcom, according to the wikipedia page for VideoCore, did however make a couple of Cortex-A9/A7 SoCs that made their way into low end 2012-3 era Android phones.
Perhaps a startup might recycle old Broadcom-based HTC and Samsung models in collaboration with the RPi team.
A RPiZ with a mobile data connection but no screen is still a potential IoT platform etc.
Openmoko with its bespoke board lived or died by how well it performed as a phone.
Just look at what people are doing with RPis and various radio boards.
There was one such project posted on HN recently that had a RPi Zero W hooked up to a touch screen and a mobile radio. All battery powered.
In a sense the Openmoko was too early and too bespoke. Thus driving the cost of entry for would be tinkerers through the roof.