If you need something like this, you don't need to fiddle with USB chargers
USB is available from almost any computing device, a growing number of displays and other peripherals, and increasingly in common areas. And AC-USB adapters are easy to get, spreading the convenience to every wall plug too. As others have mentioned, AA-USB adapters exist too, spreading USB's coverage to everywhere you can find AA batteries. As I hinted earlier, most recent devices have switched from AA to USB-charged built-in batteries, so a USB battery may have more utility than AA batteries these days.
Tangentially, there's a behavior pattern prevalent in the "Prepper" (aka Emergency Preparedness) community that confuses me. They take a logically-sound concept such as storing large amounts of food, and seem to separate the emergency food from every day food. Here, similarly, we have an emergency device that is not usually used. How do we know if it still works? Test it occasionally?
The USB battery breaks this pattern in that you can use it frequently and it's still useful in an emergency. And using it regularly can build a habit of keeping it charged, and you'll know if it breaks or goes bad before an emergency.
I'm not saying AA powered emergency devices are harmful, but rather our emergency devices and processes should be our every day devices and processes, whenever possible.
I have a phone like this from about 10 years ago that we leave at a family camp (super rough cabin with no utilities). With the loss of quality, power-outage survivable POTS phone service, this type type of device fills an important niche. 20 years ago, NYNEX (now Verizon) would provide dial tone for 10-14 days in the event of a blizzard or other power outage. Now you get 24-72 hours from cable/internet providers at best.
Another issue with conventional rechargeable batteries is they don't handle charging well when discharged completely. A cellphone left in a cabinet for 5 years is a brick in many cases.
In the same way. Test some, store the rest. When stored food nears it's life expectancy, test or just plain eat it.
If you're really serious, you tape shut your fridge door, water taps and flush toilet if you live in the city, open most of the circuit breakers if you don't have a generator, etc. etc. for a few days and test everything for real (then again your electrical company is likely to give you occasional opportunities to test your generator :-).
Any way you look at it, the more stuff you have that works or is edible, even if some of it fails, the better off you are, the less you have to expose yourself to whatever is out there, the more you can help your neighbors.
Saves us both time and money to have a bunch of staple goods at home that get restocked every six months or so. And in an emergency, hey, we've got a bunch of the same kind of food that we eat anyway, and have more than enough to share with our neighbors.
Outside of a supply of bottled water that gets rotated, we don't really have anything purchased purely for emergencies. Plenty of backpacking gear, but that gets used on the weekends, and we know that all of it works well.
One thing I did start doing was leaving my backpack loaded and ready-to-go near the door. Our biggest threat in Japan is earthquakes, and if we have to evacuate, having the same kit that I would carry for a three-day backpacking trip would make things a lot more pleasant.
How much is fresh or thereabouts food, vs. stuff from cans, white rice, beans etc.? If too much of the former, and you don't have a list of stuff you normally use in backpacking trips, then you'll either have to try some ahead of time, or store it anyway assuming you'll be hungry enough if the emergency comes, which I've read can be a terrible mistake.
Ditto bugout bag contents in case you have to evacuate, as you note.
The stuff I store for emergencies right now is "purely" for them, but it's all stuff I like (white rice, indica and japonica, which my normal input of is limited, then there's there is no MRE desert that isn't at least good, and most are very good to excellent, and, oh, even some of the Humanitarian Daily Ration entrées are just plain good even if a bit old: https://en.wikipedia.org/wiki/Humanitarian_daily_ration and you can occasionally find extras. Companies that pack this stuff for the government, especially MREs, will produce more in a lot than needed, to make sure their yield for the order is high enough, knowing they'll easily sell the extra to civilians, and civilian demand is I suspect getting high enough that they're now making more than a little extra).
Purely in scare quotes in that while they're still palatable, I'll buy fresher product and eat the older stuff so it won't go to waste. Ditto the Mountain House freeze dried stuff, it was easy enough to find a set that were at minimum "good", and if I haven't used them in some years I'll replace and eat the older items.
I also do things like maintain a minimum of a year of various specialized dry goods, like allergen free shampoo and soap, and replenish every April and October when the weather is moderate here in the US. This way if one of these products becomes unavailable (or in the case of one soap product, contaminated with perfume, at least two lots), or if there's an emergency, I'll have a minimum of six months to procure replacements, and in the latter case I won't have to be scurrying around with everyone else to find the necessities of life, and as we've both noted, can share with neighbors.
I also do things like maintain a minimum of a year
A minimum of 6 months, a maximum of 1 year when I replenish every April and October.
You're not wrong, and this mindset isn't limited to people that are preparing for either a big earthquake (that'd be me, since I live in Japan) or the zombie apocalypse (much lower down on my priority list).
As a case in point, I personally know plenty of people that have bought pepper spray for self defense, but who have never once actually tried it out to see how it worked.
It's the Magic Talisman Of Protection mindset; e.g., "this object will protect me".
But if we move to the domain of guns, I'm AMAZED at how well and responsibly many many people in the US with little or no training use them in self-defense, which I attribute at least in part to centuries of ergonomic improvements, which for me and handguns reached their apotheosis in 1911.
Even if such an attitude and approach is appalling to someone who's father started taking him hunting with him when he was 3 (ditto all my siblings), started shooting a BB at 7, (safe) dove hunting at 11 (ditto my brothers), high school JROTC rifle team 16-18, etc. etc. etc., and never advise this, I can't argue with the empirical results, and the relatively low number of bad outcomes, all most all of which are due to misunderstanding the law and morals those are based on rather than bad gun handling.
Having a spare battery in a single waterproof package is just more convenient for me. It lives in my backpack, and I don't need to care when the rainy season hits.
Plus, since I use my phone as both a camera and a GPS, I can take the Novobeam with me while backpacking to keep my phone topped up.
Between spending money on a gimmick mobile, and spend money on a USB charger to keep using your everyday smartphone, the path of least resistance always wins.
Particularly when the USB charger costs a fraction of what the gimmick phone is sold for.
But there are USB chargers that run off those, too, so you don't really need a special phone.
Personally, I carry an Instapark Novobeam 3000 in my backpack. It's about the size of a roll of breath mints or a small flashlight, is rechargable, has a waterproof cover, and costs about nine bucks on Amazon.