Earlier quoted context omitted.
> ” How much extra cost does a hard on/off switch add to the bill of materials?” BoM cost is not the whole story; a physical switch can greatly complicate an assembly, and may require hand-soldered wires. In addition to those cost-related factors, external switches are usually quite ugly, and getting them right can be very challenging, so it’s much easier to just omit them.
I think this is special pleading. At the volume we're discussing a surface mount flow solder on off device is not going to kill the BoM, and could be designed not to be ugly. It's no different to any other bad design. It could be better. All phones and computers can be completely powered down. They're hardly ugly switches.
iPhones (and I assume most Androids as well) can't be completely powered off. Neither can Macbooks. In fact, I'd guess that most modern digital things with non-replaceable batteries can't be powered off.