An interesting thought, but is it true? I cant really think of people complaining of arm fatigue from holding up a ~150-200g phone -- the effort of holding up your arm and hand alone are going to be substantially more than additional weight of the phone.

I also dont really see any evidence of phone manufacturers decreasing weight over the years, at least with a quick review of Apple's iPhones, which have definitely increased in weight over the past decade.

And increasing the length of a phone use-session is important, because it "unlocks" experiences people wouldn't otherwise bother with, that might then increase their total satisfaction with the ecosystem, and so increase brand loyalty. For example, some people just don't want to read books on their phones... but hand them a lighter phone, and they suddenly change their minds. They don't realize that that's the difference; it just is. And this might be the first device that got them to read e-books—which means they now associate the positive feelings of reading books in situations they wouldn't have otherwise been reading, with your device.

> which have definitely increased in weight over the past decade

They do increase, but only to a point; far less than if it weren't an optimization problem between increasing battery mass to power the features promised, vs. decreasing total mass.

Also of note is the fact that as phones are made more durable, people use cases less, which decreases total in-hand weight. So switching various pieces from plastic to aluminum, or one type of glass to another, may be a win even if it increases the base mass of the phone, as long as it decreases the probability that the user will feel the need to buy a case.

The iPhone X is significantly heavier than any previous (non-plus) iPhone, which is by far my biggest complaint about it.