Since getting my Pixel XL I have a noticed I rely on external battery packs less as I often have access to a power socket for at least an hour or two when I travel.
I can count on running off the battery all day as long as I can get it plugged in while I fetch a sandwich for lunch.
If I plug mine into a 2 amp usb A charger it says that it is not charging, but does that mean it is drawing zero watts, or only that it is only drawing 10 watts? It would be nice to charge it overnight if only a phone charger happens to be around, or even just slow down the discharge rate while using the laptop.
Forcing user to learn all kinds of magic rules for how to "treat their battery" seems very old-fashioned and strange.
What you have to consider with fast charging is that it degrades the batteries faster and produces more heat.
I can understand if someone says they want even lighter.
I've only used my battery pack four or five times in the year and a half I've had my 6s. Most of the time I'm not far away from a plug - either at home, in my car, at work, etc.
But when I travel, a slightly bigger battery wouldn't be enough. I would want twice the battery life - something I can get by using a battery pack - but I probably wouldn't get from a slightly larger iPhone.
If you have to optimize, why optimize for the edge cases instead of the most popular case and give options for those times people need it.