From the manufacturers point of view phones not being future proof is a feature, not a bug, that way you'll upgrade to that new shiny item which will keep the profits rolling in.
I don't think a better AI chip will be a convincing argument to change a phone one year later.
With each iteration of phone, they add a shiny new feature that is ONLY available on the newer model.
With Steve Jobs gone, Apple is more and more seen as the emperor without clothes[0] that it is.
[0] I.e. yet another rebrander of Chinese goods with marginal differences from other franchises
Aren't all the iPhone chips ASIC's with the main one custom by those hardware people they acquired? Seems to be default expectation for whatever they add next. They sure as hell have the money, too. :)
DSPs are not nearly as good at matrix math as a GPU and the phone already has one of those. DSPs are typically good at signal processing in a fairly limited domain, they would not be calling it an AI chip if it was just a DSP.