iPhones were always designed and meant to be used without any case or screen protector. Millions and millions of dollars in R&D just for this alone. Apple's ridiculous performing IP68 ratings says it all. Apple knows they will get scratched and nicked over time--that's the goal. When Steve Jobs designed the iPhone 4 he said it was like a Leica camera. Have you guys ever seen a used and worn Leica camera? It's gorgeous.
The swiping gestures, the way the glass meets with the metal bands, the buttons clickiness, the cold steel and glass when you pick it up. These are things Apple want people to experience.
Unfortunately, the phone case craze started around the iPhone 5 and beyond for some reason--oh I know the reason: resale value. I guess people bought into phone case marketing that these phones are ridiculously fragile that they warrant a case--yet a majority of people who have them in huge Otterboxes don't really work in construction or any environment where they're dropping phones or scratching them up.
99% of the time, youre not going to be dropping your iPhone or causing deep gouges of scratches by using your iPhone, putting it in your pocket, putting it on a table, etc. And if you do drop it? It's a 50/50 chance it'll either shatter or nothing happens.
Having a mint, pristine, museum quality iPhone for resale doesn't net you as much money as you think it does over a normal used iPhone. I've tested this and I've only netting an extra $50-80 bucks. Not worth it IMO.