Many folks (most in the last couple of years?) are firmly entrenched in the Apple ecosystem. Wanting a nicer camera and nicer screen, while not having to worry about migrating data and apps, forces the 'lighter/thinner' option on people. People moving from an iphone 5s to a 7 aren't necessarily "voting" for a thinner phone. They are more likely "voting" for a better camera or more storage, but are forced in to 'thinner' as a byproduct.
I've never met anyone in the last 5 years who's upgraded to a new phone based on their current device being 'too thick'. Camera, storage, speed and screen size are factors; thickness has never been a motivating factor I've heard from people.
(And to preempt the typical responses to this: I know battery cases exist, but they're a substantially inferior solution.)
If you had both choices before you in the store, who wouldn't start to wonder if they'd regret the thinner/lower capacity choice?
Imagine a "Black Edition" of the iPhone that's thicker, has a headphone jack and an integral rubberized case. The aesthetic would be similar to military style watches.
Imagine a thicker MacBook Pro that doesn't require dongles. Don't announce them initially but show it in some hacker movies. Make them special order initially. I can dream...