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The radio firmware could be executed on different CPU than the Android kernel and communicate with a standard protocol ?

That's more or less what happens already.

It is but if the radio driver was open source then the interface with the radio SoC would be documented.

Right now it is the worst of both worlds. Closed interface for a closed undocumented chip able to execute arbitrary code.

The interface isn't closed anymore, is it? [1]

Drivers don't need to be open source to interact with GPLed code.