I used to be an every two years type of person for each phone (I usually had anywhere between 3-4 at one time) until the pandemic completely interrupted the upgrade cycle of my 11 Pros. I still have them - Turns out if you turn into a hermit and only take calls thru Facetime on your iPads for three years there's actually no need to upgrade your phone.
I did consider the 15 Pro last year but I'm still not out and about anywhere near as often, and when being out and about I actually felt minimally inconvenienced - partly due to my phones mostly being used for phone things, not expecting to do everything on them, and also because despite being 4 years old at this point the batteries are still at 90%.
And now, I'm just thinking I might as well ride it out for the full 5 years and get 16 Pro's. I might even reduce to two phones (third phone is Android) and just go down to the one Pro.
Keeping a phone for 4-5 years is something I've never done until now, and I'm pleasantly surprised by how little of a problem it actually is - it's still totally usably fast. Definitely wouldn't have been the case e.g. ten years ago, but you can really feel how things have plateaued. So there's a very real chance I might also hang onto the 16 Pro for about the same time.