I started using cases after having a Pixel 2 XL shatter in a 1 foot drop onto a granite countertop, but started going case-less again with my most recent iPhone 15 Pro (ironically, after the horrible quality Apple case I mistakenly bought with it disintegrated).
It's been a year so far of completely non-cautious use and my general take is:
* The screen still scratches; I leave my phone in my pocket or a bicycle saddle bag and it definitely has some damage. However, it is only visible with the screen off, and it turns out not to bother me a single bit.
* The frame of the phone seems almost invulnerable; even with a case many of my older phones got dinged corners, and this one is perfect.
* Overall, the only functional issue I have had with going caseless has been the propensity for the protruding cameras to pick up dust and fingerprints. I find myself having to wipe them even more frequently than I did when I had a case.
I suspect that a case still would have been a net-positive fiscal investment, I'll probably have to sell this phone in "good" or "fair" condition rather than "factory new" like a phone with a case and glass screen protector for its whole life, so I'll lose $50-$100. But I like using the phone on its own. The side and action buttons finally function as they should and the size benefit is appreciated.
Anyway, I'm now a caseless fan and this makes me very interested in the new Air. I think the need for cases is perhaps overblown and especially in a premium market like high-end iPhones where the consumer probably isn't as sensitive to aftermarket value, no-case isn't as rare or foolish as it would seem. For that reason, I doubt a bit that Apple are assuming the average person has a case on their phone. I'm sure they do focus groups, testing, and have some degree of telemetry to understand the case-vs-no-case debate in detail and I strongly doubt that the conclusion is "we assume there will be a case so we will do X".