I know you don't think much of me and this might come over as pretty asshole-ish, but have you used an iPhone and an iOS in the past years?
I don't use many apps on my iPad, but I can't think of any big universal iOS app that has the same or similar layout and controls on both devices. Not to mention that most big apps bring their own UI and ignore the iOS one.
But again, since I don't use many apps, my perception could be very wrong.,
You're right the best apps are totally re-worked for iPad usually, I'd argue about different controls but I'd be splitting hairs, at the end of the day it's totally possible to ship a universal app that has different UI for iPad and iPhone because it's the same SDK. You could make that work with W8, WP8 as well I'm sure.
The difference is that when you release an iphone app, you know that if people download it on iPad and it will make just as much sense as it did on their phone because the functionality and design language are identical. They'll know how to interact with it, and even though it's scaled up and looks a bit crap it will visually fit with the rest of the platform.
On Windows 8 you have:
-2 missing hardware buttons
-A system designed to be used landscape
Both of which are solvable but make you start thinking "hmm is this really worth it" and then you have the UX guidelines for apps that specify among other things:
- Vertical scrolling is bad
- Top bar for app navigation with your custom icons
- Bottom bar for interactions with the page your looking at
- Right bar for application settings
etc etc
I don't see how any of those things could be inferred and transferred from your WP build so you're going to have an app that doesn't navigate or function the way Windows 8 users (have been taught to) expect. That's not just a rework of the front end, it's a complete rewrite.
Basically, MS broke metro with Windows 8, if they want to unify it's a much bigger job than just having 1 sdk.