I don't recall, but was durability an issue for the 6s phones back in the day? Are they using different aluminum on the 17 pro series?
I certainly had issues. My 6 (standard size) was my first iPhone and I kept it in a case. That didn't prevent damage. Dust scratching the back of the phone when it got caught between the phone and the case. Screen delamination from the bottom right corner where the aluminum bent away from the glass, causing touch sensitivity issues. Minor bends despite the case and no abuse of the phone (never dropped, etc).
I decided
surely Apple had fixed the problems by the time they got a few more generations in, so I bought an 8 Plus to replace it. I decided not to go with a case, because I thought perhaps some of the delamination issue was due to popping the phone out of the case periodically to clean out the dust. That phone didn't have as many micro scratches on the back, but it did eventually succumb to bending & partial screen delamination despite being careful with it. Touch sensitivity didn't seem affected. In the last month before I upgraded, I did drop it a short distance and got a solid gouge in one corner and minor damage to just the edge of the display's bezel (no cracking).
Which leads to my iPhone 13 Pro Max, an absolute
tank. I used it caseless for its incredible premium feel and never had any problem with it, even though I was less careful with it than the fragile aluminum phones. It got dropped on concrete and asphalt and never showed a scratch or a dent. It held up to years of hard use and looked like it was still in near-original condition until it took a feline-induced tumble off a railing and down a flight of concrete stairs. That caused the (non-Ceramic Shield) back glass to crack badly, but not break. The rest of the phone, surprisingly, still looked fine aside from the tiniest chip in the front display which was practically invisible. No bending or denting of the metal. I suspect that if the back glass had been Ceramic Shield, it would have been in better shape like the display was.
Unless Apple has massively upgraded the quality of aluminum they're using, I imagine that the iPhone 17 Pro models will suffer the same issues over time as other aluminum phones. However, since the back glass is now Ceramic Shield, I suspect that glass will hold up better than the previous generations of Pro phones, but the surrounding frame will wind up bent, scratched and dented with normal use - case or no - although after having tried it both ways I'd recommend a case on an aluminum phone due to the softness of the metal. You just might have to deal with the back getting some microscratches and likely delamination, particularly if you're taking the case off periodically to clean the phone while trying to reduce that scratching. I would have a lot more confidence in the durability of the iPhone 17 Pro if Apple had given it a steel or Titanium band around the edge, even if they did use cheap aluminum for heat conduction in areas of the back that are not glass.