It seems reasonable they might next apply this to the iPhone - and it seems the different battery sizes you suggest would be easily accommodated. (Unless there's another reason they want to constrain battery sizes).
EDIT just thinking of Christiansen's ideas on evolution of products, basically from "integrated for performance" to "componentized for customization", it does seem that iPhones are now overpowered for... well... phones (they are pretty much last-gen console/low end PC level now, or will be soon). Therefore, people might soon stop being willing to pay for even more performance (maybe). This is a problem for Apple, because they don't like commoditized products, not from a margin point of view, not from a building cool new stuff point of view. A way out of this is cool designed customization - it's something that Apple likes, and has great margins (like Beats).
The race to idle means that we still have room for more powerfull mobile CPUs. It's just that the extra power will appear as extended batttery life at the marketing material.
Anyway, I think the current generation is very near "good enough", people may switch once more, but the fast replacement of phones are getting to an end.
Marketing wins, actual practical use of the product loses.
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.
It really seems like your usage style is rare. I've never had an issue with my iPhone's battery life. It easily lasts two days without worry. On the other hand, I've never thought, "This phone is too thin." Apparently, Apple thinks that the best compromise is to keep the same dimensions and let others manufacture charger cases.
I suspect my comment was downvoted by people trawling my history after seeing https://news.ycombinator.com/item?id=8879389 and disagreeing strongly. It may just be confirmation bias, but this isn't the first time I've noticed my older comments getting downvoted after a comment of mine gets a lot of eyeballs.
I realize karma means nothing, but this behavior harms the quality of HN. The mods may want to consider writing a detection/mitigation feature to ignore such downvotes.
I think many people would want something like an iPhone 6S with the option of having a bigger battery and built-in case at the expense of a few mm of thickness. Basically, make the back removable (even if just when ordering or at Apple stores) and make a more rugged version which also contains a second battery. The fronts and guts can all be exactly the same for manufacturing efficiency, then the wrap-around back can be swapped out to change color / if it gets smashed or bent / if the user wants to replace the battery etc. I really think this could work while retaining their fit and finish + manufacturing efficiency, and it's surely make consumers happy. And again, I'm not talking about a user-replaceable part, just an ordering option or possibly a paid in-store swap.