I could maybe see them adding pencil support to the iPhone Pro. Otherwise I'm not sure why they should increase the thickness, power draw, and BOM for.a feature that maybe 0.1% of the iPhone audience would ever use. (And that's being generous.)

And Apple is in yet another no-win scenario with the iPad calculator. Provide one and people will complain about how they're preventing poor Apple developers from competing on the iPad. Or let the developers flourish, but then get snark from medium authors with too much time on their hands and nothing else worthwhile to write about... /s