This concept of a cheap spare cellphone with a 15-year charge on one AA battery sounds like a good idea. When I travel in Europe I frequently change to local SIM cards. It would be good to “park” the home SIM in something that doesn’t need charging, just in case an urgent call comes through. I would buy one of these in preference to carrying a spare phone and charger.
Incidentally, this just proves the point I have made repeatedly: If you want a long battery life you can’t have all the goodies and speedy bits. This SpareOne phone doesn’t even have a screen.