The cellular, Wi-Fi / Bluetooth and NFC radios do receive firmware security updates. GrapheneOS isn't going to support devices without proper security support, which includes ongoing maintenance and security engineering / research for the firmware and drivers.
Focusing on the cellular baseband is missing the bigger picture. There are dozens of computers in modern personal computers running their own operating systems. Cellular basebands are very directly comparable to the Wi-Fi SoC. It's a mistake to think that the same things don't apply to Wi-Fi, especially when on so many devices it's much less contained than the cellular baseband. I'd recommend checking out https://googleprojectzero.blogspot.com/2017/04/over-air-expl... which is about exploiting the Wi-Fi SoC older generation device, which then provides full direct memory access since it wasn't meaningfully contained by the IOMMU. It was a configuration and driver coding issue, as the hardware was entirely capable of containing it but was unfortunately not set up to do it.
Focusing on the cellular baseband is missing the bigger picture. There are dozens of computers in modern personal computers running their own operating systems. Cellular basebands are very directly comparable to the Wi-Fi SoC. It's a mistake to think that the same things don't apply to Wi-Fi, especially when on so many devices it's much less contained than the cellular baseband. I'd recommend checking out https://googleprojectzero.blogspot.com/2017/04/over-air-expl... which is about exploiting the Wi-Fi SoC older generation device, which then provides full direct memory access since it wasn't meaningfully contained by the IOMMU. It was a configuration and driver coding issue, as the hardware was entirely capable of containing it but was unfortunately not set up to do it.