OEM support for the device is needed because an alternate OS cannot provide firmware updates otherwise.
Firmware and drivers can be made open, just as other software can be made open. It's really just a matter of incentives. In my experience, law tends to be a pretty effective incentive.
If the bill of materials included the legal requirements discussed here, then a component supplier would either start producing open firmware/specs, or they would lose that market to another supplier.
Obviously, Android would not be the only project/product affected by such a legal change.