Comment by iamnothere

The Linux USB stack improves over time, and besides, implementing it with USB makes it easier to implement hardware toggle switches. (Cutting power pins to the USB modem is like unplugging it.)

Edit: I’ll add that I think smartphone “security” is almost impossible to achieve, given the complexity of everything and the opacity of modem vendor stacks, which is why I just assume endpoint compromise. I use my phone rarely and with toggle switches normally “off”, and I don’t consider it a secure device or use it very often. If you believe that a secure phone is possible, however, then Graphene is definitely a better fit than a Linux phone.