>Obviously we don't have that. But what stops an open firmware (or even open hardware) GSM modem being built?
The same thing that stops you from living on a sea platform as a sovereign citizen or paying for your groceries with bitcoin. Technically you can, but practically you don't.
If you want to sell it commercially, you can opensource all you want, but the debug interface and bootloader integrity would have to be closed shut for production batch.
At best, you can do what the other comment refers to -- instead of using the baseband as a source of root of trust, make it work like wifi modules. This of course comes at a cost of having a separate SoC. Early motorola smartphones (EZX series) did that -- Linux part talked to the gsm part literally over usb. It came with all kinds of fun, including sound being khmm... complicated. I don't remember whether they shared the RAM zo. You don't want to share you RAM with a funny blob without reading fine print about who sets up the mappings, right?
Figuring out all of that costs money and money have to come from somewhere, which means you also have to resist the pressure to not become part of the problem. And then the product that comes out is 5 years too late for the spec and 1.5 times too expensive for the vague promise of "trust me bro, I will only blow the e-fuse to fix actual CVEs".
There are some open firmware, or partially open firmware projects, but they're more proof-of-concepts and not popular/widely-used. The problem is the FCC or corresponding local organization requires cell phones get regulatory approval, and open firmware (where just anybody could just download the source and modify a couple of numbers to violate regulations) doesn't jive with that.
Why not?
Obviously we don't have that. But what stops an open firmware (or even open hardware) GSM modem being built?