[rsync]

"Is that really where the goalpost is though? I thought everyone accepted that the modem itself would always be proprietary, and that was OK as long as it had a killswitch and no ability to directly interfere with the main processor or RAM."

Most of the time when we speak about the baseband, we are talking about the telco network interface between your handset and the carrier ... and so it's tempting to think of the baseband as just some black-box modem that you can firewall yourself from.

However, the baseband processor also performs a number of real-time voice quality / noise cancelling / audio functions that really have nothing to do with the network interface, but are very important depending on how sensitive you are to call quality and things like echo, etc.

This is unfortunate because, like you, I would like to just wall of the baseband and use it as a modem and forget about it, but that perceptible difference in call-quality between "actual carrier" calls and VOIP calls is due to the baseband.

I believe that is all still true in the 2019 LTE era ...