My wife just upgraded from 12 Pro to 15 Pro. I feel the upgrade is decent to justify it, but when I compared to my 13 Pro, I feel far less compelled to upgrade. Yes, it is your typical spec bump, of course things will be slightly better, but I feel the changes are soo incremental, that upgrading from 13 Pro to 15 Pro might feel like waste of money for many.
Sure, some might be swearing by USB-C, new slightly improved design etc, but if none of these matter or seem important to you, then I would save the money and keep the existing phone or pick an older iPhone model on sale.