If you turn on airplane mode it disables the cellular modem, also if the modem is compromised it can't affect the OS because it's isolated via IOMMU. The only thing a compromised modem could do is ignore airplane mode commands.
If you're going to keep airplane mode on 100% of the time, why would you have a device with a cellular modem? If you're going to turn airplane mode off, your backdoor is open. It's just a thing we have to accept for now if we want to have phones. Someday we'll have better hardware.