I've held onto my iPhone 8 for 6 years because I didn't like the size, but particularly the weight of the newer phones. I'm considering some flavor of iPhone 15 Pro because it would eliminate that issue for me.
Aluminum might be lighter, but it's also easily deformed, which was a huge problem with the iPhone 6 - not just with bending, but with deformations after being dropped causing issues with the screen (touch disease), which cost apple a lot of money in repairs and replacements. Aluminum is a bad choice for a chassis which is why they moved to stainless steel in the first place.
Titanium is a superior option because of it's strength and weight, but it's hard to machine and it's expensive. Is it necessary? No. But periscope cameras, smaller bezels, and a cut out in the screen are also unnecessary, hard to make and expensive. It all comes down to what they can include for a price people are willing to pay.
I'd prefer the titanium frame and periscope camera at the same or lower price point as the 14, even if it meant a smaller screen with bigger bezels and no cutout. But I don't design the phones.