You’re right, of course, and that’s what I will do. It’s normally good enough for my needs.
I assume the reason behind this design is limited space and a wish to avoid an array of controls. It is a tough problem when there are so many independent things one might want to turn off and be sure they’re off: two cameras, mic, accelerometer/gyro (can be used as a mic), GPS, wifi, cellular modem, bluetooth, USB.
There are several threads discussing kill switches, e.g. this one with actual design suggestions:
I wonder if the 3.5 mm mic/phones jack is still present. If it is, then the mic could also be killed using a plug with no cable leading out of it…
And a lens cover would be enough to kill the camera in my case, but maybe that would add too much thickness to the phone if made as a sliding part of the shell.