The only other way to do it is to have an SoC with the modem integrated on chip. The issue with that becomes that the firmware and usually drivers force a certain Linux version (my tablet is stick on Linux 3.10, my phone is on 4.19 and I'd be amazed if it was ever upgraded).
With this way, they can mainline the entire set up and not force non-free dependencies.
If it's going to run a Broadcom/Qualcomm binary anyway to function, what do you really gain?
Someone who's concerned about this level of eavesdropping I would think shouldn't bother with cellular devices at all?