And while you can argue about whether thin laptops are necessary, for people who carry their phones in their pants pockets, an extra couple millimeters gone is genuinely a meaningful difference.
There's no propaganda there.
Give me back the old phones, removable battery, more thick, but more compact in the end, and more easy to carry around. The bigger screen it's in the end useless to me.
I doubted this so I looked up dimensions. Samsung Galaxy S6 from 2015 is 6.8mm thick, with removable battery. S10 from 2019 is 7.8mm thick. So your theory doesn't seem to match reality.
https://en.wikipedia.org/wiki/Samsung_Galaxy_S6#Battery
In any case, it's apples-to-oranges and doesn't mean anything. Cell phones can have functionality added that takes up the difference in thickness -- e.g. keeping the phone the same thickness instead of it getting even thicker.
But main point is, they're thinner basically by definition. There are necessarily more layers of materials.
Of course there are some tradeoffs but it just doesn't seem so significant that EVERY brand refuses to offer even one model with a removable battery. There are definitely some people that want it despite the tradeoffs. As other commenters said, people have huge phones now and use cases. So I think manufacturers have a special interest in maintaining the situation for planned obsolescence.
I've had an iPhone battery replaced 3 times now. It's not a big deal to have the store do it for me, nor is it that expensive.
The bigger use case for swappable batteries is to have a spare, but these days people just carry an external battery pack with them that's the capacity they need, which is far more flexible (hold 10 full charges if you need, not just 1!) as well as not tied to any particular model.
So I just don't see any special interest -- it's just giving people the thinness they want.
[Citation needed]
My "thin" phone double in volume with the require case, so "thinness" is really a joke...
Hint: it's not the non-replacable battery why people upgrade phones.