I would argue that the battery pack market has little to do with whether or not folks think their phones are too thin. For instance, in day-to-day use I don't recall ever plugging a device into a battery pack. But I own two of them. Because once in a while I'm in the middle of, say, the Yukon or Alaska and haven't seen a power outlet in days, but perhaps would like a movie in my tent or to write a blog post. (I could charge from outlets on the motorcycle, but I've got enough crap plugged in the way it is.) So most of the time those packs sit on a shelf and are no reflection on whether or not I feel the iPhone battery is adequate for normal use.

Not everyone hangs in the same circles, but lots of (for example) motorcyclists buy these for general "plug in at camp" use. Run a small light, charge the GoPro, and some do use them to charge a phone. Hell, some of them use them as jump start batteries which is about as far away from anything related to cell phones as I can imagine in the portable power market.

In summary, quoting battery pack market size merely reflects the desire for folks to have portable electrical power, for whatever they feel they might need it for.

Power banks are convenient and I'd rather have a power bank and a thin phone than a really thick phone and no power bank. Thin when you want it, bulky when you don't.

The 3.5mm headphone market is also massive but people don't want 3.5mm headphones, they just want headphones. You're confusing demand for intrinsic virtue.