For CRUD apps, the API spec and UI mockups -- or even a photo of how it looks on the already coded platform -- would go a long way. That's exactly the kind of well defined task work LLMs do well with. It should be possible to automate a lot of the equivalence testing too.
Is there still an excuse for "maybe we'll add Android someday" or "not enough Mac/Linux users"? And is there still a justification for not building those less-used flows like password reset into the iOS app instead of throwing up random WebViews?
For those apps that do have non-trivial logic on device, LLMs have shown a lot of promise at rewriting to cross-compiling-is-easy languages like Go or Rust.