iPhone Pro prices have only increased once, for 15 Pro by $100 which continued with 16 Pro, despite higher inflation over the last 4 years, so they've likely lost margin. If they're trying to change that they either increase price or lower cost to make, and they have likely done surveys that show that Pro buyers buy it because of cameras, screen etc, more so than the frame material, and considering Pro Macs and iPads don't use Titanium, they've made the choice to go all aluminum.