This is by design of course. The only major platform that requires a specific minor platform from the same vendor to target, at least that I know of. Apple knows how to make money.
> You can't build iPhone or iPad apps on iPhones or iPads yet, so a decent chunk of that revenue currently relies on the Mac as well.
This is both entirely true and still manages to miss the point. Yes, Apple keeps the Mac around exclusively to accommodate the creation of iOS apps. No, they are not financially incentivized to create new categories of hardware that cater toward productivity when instead they could lock them down and milk that cow for all it's worth via the app store tax.
Even without that, calling a $30B business an "insignificant historical afterthought" is a bit of an exaggeration, no?