Google does the same with its Nexus devices[1] in that you can build a completely open-source version of Android except for certain hardware-specific binary blobs.

[1] https://source.android.com/source/running.html

So... You can build the complete OS... Except for all the hard driver parts.

The hard driver parts are for things like LTE, GPS, GSM, wifi, camera, etc. which the device manufacturers haven't chosen to open-source. I don't see how it's relevant to Android itself, any other OS would have the same problems with that hardware.

As does Google. Or has "fastboot boot" stopped working on Nexus (and many other bootloader-unlocked) devices?

> Hell, you could boot a kernel over USB, and Palm gave you the tools and commands to do it

So does Google. All Nexus devices allows you to do "fastboot boot kernel.img".

Other devices with unlocked bootloaders supports/supported this too (like my old HTC One M7), using the same standard Android tools.

It's sad that what you're describing sounds scandalous today.