There are three points here that support each other but are not the same, UX, Software, and Hardware:
- No one wants to run iOS apps on a mac. This is about normalizing the internal apis that you use when building apps. Currently there are entire systems that do similiar things but not quite on both platforms. Having just ONE color lib would be very nice (for example).
- Shared widgets? Sure to some extent, but not a primary thing, I could see the mac UX adopt a number of touch controls, but even more useful would be enableing reuse of the controller/model logic across platforms. If the controller for your ‘email field’ worked on both platforms it would save a lot of time.
- XCode already does cross compilation. If your had macARM as well as macIntel targets it’s mostly a wash for a lot of developers. Apple also already has the app store that strips/rebuilds bundles for the vairous target platforms so size will be handled.
I think you should look at switching to ARM just like switching to Intel. They are looking to get better performance for less power use. Performance that can be unlocked by custom chips tied directly into the software layers. Added benefit of less power use leads to even better device form factors.