Just as we were in a period where every new A chip brought significant speed advances from ~2010-2018, I think we are now in a period where every A chip will bring significant NPU advances. As others have pointed out, Apple Intelligence is still not feature-complete from what we were shown at WWDC and a lot of the features that have rolled out are not working that well yet.
My suggestion would be to use ChatGPT / Gemini / Claude etc for now (even though their features won't be usable in the native Apple apps) and look towards the iPhone 17. I think it's highly likely that Apple will lock some new AI features behind the A19 chip, so if you're interested in upgrading primarily for AI I would wait and see what the iPhone 17 series brings.
Even NPU in desktop cpu does basically nothing now, all serious AI stuff must use gpu. What can we expect from a much smaller phone chip A19 to A whatever?