So I found a remarkably simple and obvious solution: I spent £50 and bought an external battery pack, something like http://www.amazon.co.uk/RAVPower-15000mAh-External-Generatio... . It charges overnight, weighs not all that much at the bottom of a shoulder bag, but on those crucial days when battery is getting slim, I can just pull a cable out, plug it into my phone, and bingo: the phone is charged.
It's capable of charging an iPhone several times over so easily solves the problem for you and even some of your friends.
Isn't it more like "Bingo: now I have a battery pack dangling from my phone for the next few hours"?
I used to own a battery pack too, back when I had an iPhone, it was a required accessory unfortunately. Since then I've owned a Samsung Note (1), LG G2, and Sony Z3, and all three of them last 2-3 days, and I have no need for a battery pack.
Even if size was a concern I could purchase a Sony Z3 Compact which is "iPhone 6 sized" but has twice the battery life or more.
Back in the iPhone 4 (ish) day the iPhone had "impressive" battery life. However Apple has stood still while everyone else has continuously improved. Now most non-Apple phones (Galaxy S5, LG G3, Sony Z3, Note 4, Windows Phone, etc) last substantially longer than Apple unless you're buying an unbranded piece of junk.
Not to mention it's about the same size as the iPhone 4. Great battery life, packed into a small handset.
Only Apple makes phones without removable backs. Anybody else wouldn't be able to sell their phones if they removed that feature.
Marketing wins, actual practical use of the product loses.