Purism is never going to end up with fully open source baseband firmware. It's not going to happen because the radios are subject to several regulations which means customers can't be able to modify that firmware. There's going to always be a trust hole.
People should push for open source as much as possible. At some point it will be easier to lobby for the new regulations when everything else is fully open source. See also: https://forum.pine64.org/showthread.php?tid=11815
The regulations around radios exist because the spectrum is limited and emissions propagate over a wide area. You and I (assuming you're in the US) have the same standing to use radio spectrum. If I go and modify my phone's firmware to increase the power output I could literally jam communications from your phone.
It's very different than if I modified the firmware on my hard drive or UEFI on a PC. I might fuck up my stuff but it doesn't affect you. I can fiddle with my hard drive firmware all day but I'm not going to block a 911 call you're trying to make.
Also a company giving out modem firmware is an exception and not a rule. It re-classes the device as a hobbyist/experimental device and if they go traipsing around with it they could potentially face fines (unlikely but possible).
Again it's not about lobbying it's about a limited spectrum and people being stupid/assholes not realizing or caring their pocket radio affects others. You live in a world where shitheads try to make their cars louder on purpose and you can pick up dozens of WAPs because everyone sets the power to the highest number the interface allows.
Using wrong spectrum and jamming communications of others can be illegal without forcing proprietary firmware. It's like making all cars illegal, because someone blocks access roads for a fire brigade.
It's not forcing proprietary firmware. The firmware could be entirely open so long as end users couldn't freely modify it on their devices. Competition between baseband manufacturers drives them to keep firmwares proprietary.
Every radio regulation agency on the planet? Most radio hardware is capable of operating outside of regulated limits. The device firmware is usually what keeps the devices running within their regulated limits and gets those components licenses to be sold. Anyone selling regulated devices running outside of their regulated envelope faces fines and even criminal charges.
Cell phones only work because the millions of devices run within strict limits and behave reasonably. There's not a lot of difference between a properly operating radio and a radio jammer. Purism isn't going to find a baseband vendor that's going to risk their licenses by allowing for open source firmware.
In the US a baseband processor's entire software stack that controls the radio front end must be certified. They'll also have the modems to talk to the cellular networks. BPs use their own CPU(s) and an RTOS firmware that's FCC certified.
This is why a baseband processor is a fully separate component from a device's application processor(s). Since the AP doesn't talk directly to the radio it doesn't need to be certified and can be updated without recertification. The BP can also get certification and any manufacturer using that BP doesn't need to re-certify it. The interfaces are also such that the AP can't (or shouldn't be able to) tell the BP firmware to boost the output power above legal limits or something.
Radios that have "open" soft modems don't typically have fully software controlled radio front ends. The radio front end will have its statutory limits baked in electrically or have very limited software control. The modulation on the back end isn't as important as the front end. Broken modulation just means you can't talk to anyone, an overdriven transmitter is effectively a radio jammer or can give someone an RF burn.
Part 2 covers recertification of changes to radio equipment (everything touching the front end of the radio). Part 24 cover broadband PCS while Part 15 covers WiFi and Bluetooth since they're ISM band components.
If you're actually interested read the regulations and look up some FCC IDs for devices.
