You really believe Apple will give their products a significant update in anything? Just look back the last 5 years and you will notice that they bring very incremental updates year on year to all their product lines. In 2024, they are still selling you Macs with 8GB of soldered RAM and 256GB of storage, while claiming it is not a problem at all. In their own words, 8GB on Mac is like 16GB on other OS/ platforms. May be you can get 16GB as an upgrade, but will cost a lot more.
In your case, I do think it may be worth upgrading your phone since the battery is expected to be terrible now. But if you are getting the Pro/ Pro Max series and if the price is better on the older iPhone 15, I see little reason to get the iPhone 16 series.