When the hardware is complicated enough that the software required to run it al all would take many millions of dollars to replicate, hardware freedom alone doesn't cut it. Just like a modern processor needs mountains of microcode to do anything you'd actually want. And that's without companies needing to obfuscate their hardware to avoid interoperability they don't want.

In practice, a whole lot software would have to be open source too so that the hardware is reasonably usable. The layers you'd need to let an iPhone run android well, or a Pixel phone to run iOS are not small.