I keep my devices for a long time. I’m currently using an iPhone 8 and have a 5 that sees daily use as well.
Now that Apple has switched to putting the newest chip in the 15 Pro and last years chip in the 15 would it be worth it to get the Pro when looking at using a phone for 6-8 years?
Go for the Pro. If you only upgrade every 6+ years why not get the best available option? The Promotion display is worth it. And even though you may not use some/most of the features now, in 2-3 years you may have a use for them.