For me, the most important question when considering features of a new “Pro” phone is: Does the new thing/change make the phone overall a more robust product?
When they switched from Stainless Steel to Titanium the answer was No. The titanium on the 15/16 Pro is just a <1mm layer fused to the outside of the outer band which is otherwise aluminum, fastened to the internal frame which is also aluminum. The change from steel bands saved weight, but that little sliver of titanium was basically a decorative veneer and not doing much for the structure of the phone.
The unibody frame in the 17 Pro is something never done in the iPhone. The entire body of the phone is milled from a single piece of metal. More structurally durable than any other way they’ve ever made an iPhone while also bringing huge benefits to the thermal design. It makes the phone much more robust overall. The only thing lost is the ‘exotic’ material label.
And no, they couldn’t do the unibody with titanium, the raw material alone would cost more than the phone itself. Steel would be too heavy. And in both cases, most of it would get milled away and both are much more difficult to recycle/reuse the waste than with aluminum.
If we were talking about a luxury car maker that made a change from using steel for a major component to aluminum because it brought major performance gains at the expense of a slight loss to cosmetic durability in the long run, would we still be having this argument?