The titanium iPhones, at least, are nearly impervious to scratches when dropped on concrete or asphalt from a reasonable height. I “scientifically” test this pretty often.
Because the iPhone 15 pro was significantly lighter than previous pro models, I wanted to avoid a case to get the most out of this improvement. However, I wouldn’t have even experimented with not using a case if it weren’t for the applecare+ plans that are reasonable. I’ve been surprised by the durability to the extent that I should probably discontinue the applecare+ plan.
The aluminum models might not be as durable. Compared to phones 20 or even 30 years ago that didn’t need a case, I suppose a significant difference is the density as much as the total weight or the hardness of the materials.