As someone who uses this solution to your above-stated problem, I disagree with the assertion that phones are "Not Secure." Care to back that up?
Low battery requires plugging in a cable to charge; the cable may be a surreptitiously-installed data cable. I do not trust the hardware or software stack that handles the data cable to be secure.
SMS: Not Secure. http://www.cybersecuritytrend.com/topics/cyber-security/arti...
Wifi: Not Secure. https://www.bestvpn.com/privacy-news/wpa2-wifi-not-secure/
Many apps "require" silly amounts of permissions to the phone before they'll install. Apps' ability to read what's on screen, to be notified of clipboard changes, to listen to sounds; all when not even in the foreground. On my computer, I can easily check what apps and background services are running. Not so easily on a phone.
Did you audit the Intel AME, HD firmware, chipset, audio drivers, usb drivers, and everything else on your PC?
This is just totally not true.
I'd wager a fair sum that passwords stored on an iPhone, behind a decent layer of security, are entirely better protected from physical access than on any general-purpose computer.