The userspace OS needs to be where control of any radio (bluetooth, wi-fi, cellular, nfc, uwb etc.) components happens, otherwise it would be difficult to be compliant in terms of regulatory requirements like a working Flight Mode.

Baseband firmware/processors are still making strides in terms of defending against attacks so I don't think we are at the point where backdoors are needed. More likely that undocumented AT commands, plausibly deniable bugdoors and existing memory corruption vulnerabilities are already convenient reality. However, part of that reality is also that once the cellular modem is exploited OTA it is still a massive task to traverse IOMMU restrictions and get device owner data/secrets from the userspace OS. It doesn't even grant visibility into application traffic which has been aggressively TLS encrypted since the mid 2010s.