"One OS to rule them all"

Nice try, Microsoft, misleading people into thinking that. But it's more like:

1) Windows 8 for x86 desktop systems

2) Windows 8 for ARM (no native apps, and quite different)

3) Windows Phone for mobiles (based on Windows CE)

My question is why didn't they just make WP7 for tablets? I know Win8 wasn't ready, but from some videos I saw on TIMN of Win8 running on ARM tablets, it was clear that the dual core ones could barely handle it, and only the quad core Kal-el had more acceptable performance. I'm sure WP7 would've flied on those. Why did they have to port that whole 8 GB OS to ARM when it will have no legacy apps anyway?