I basically want a pocket computer. Access to the cell network is a useful feature, but one I don't need most of the time, so why be connected to it all the time, with all the privacy concerns that comes with.
Despite the name, it's basically just an iPhone without a cellular radio (though I'm not sure if it's actually removed, or just disabled/neutralized). So you can use it to do anything an iPhone can do, it's just dependent on having access to a WiFi connection if you want to do anything over the network.
That said, sometimes you need/want cellular in a pinch, but don’t want it always on, hence the kill switch.
If you have hobbyist-level electronics assembly skills, a weekend, and a couple of hundred dollars, you can pretty easily assemble such a thing using a Raspberry PI with a touch screen, battery, and any of several commercially available cases (or 3D print your own).
I have one that I put together to use for pentesting. It works quite well.
The easiest way to meet the size constraint is to use the Pi Zero. I wanted more grunt than that, though, so I used a normal Pi and stripped the board of all the connectors except the micro USB to achieve the profile I needed.
You can't. If the functionality can be enabled/disabled via software, then there's always the possibility that malware will enable it.
I can only think of the special case where a carrier wants send a specially crafted message to exploit a flaw in FLOSS in order to implant malware that tricks the user about the status of the baseband. Still, if the carrier wanted to take that risk the only users who are protected are the ones who never switch on the baseband OS.
I don't think it makes sense to decrease usability for the off chance that some user has an idiosyncratic threat high enough to never use the baseband os, yet low enough to toggle wifi on and off.
I'm not speaking to the number of switches, merely the importance of actual physical switches that physically disconnect the subsystems.