But that's not what is being suggested here. Some people on this thread are hoping for / predicting an ARM based Mac Mini running regular OS X. That's not "better". To use your examples, it's more like Microsoft's pen-optimized Windows than like an iPad. It's more like this iPod Phone speculation than what was actually the iPhone:
http://www.ijailbreak.com/wp-content/uploads/2012/04/apple_ipod_phone.jpg
Also, while it's true that OS X and iOS are from the same codebase, that does not mean they are the same OS. A vending machine dispensing Coke, the Mars Exploration Rover, and my Linksys Router all run a derivative of Unix. That does not make them in any way interchangeable, similar, or even comparable beyond the Unix background. There's much more to an OS. Both are great OSes, but there are far more differences between OS X and iOS than similarities. Putting iOS on an ARM Nano Mini or whatever would be as silly as putting Coke's vending machine software onto my router.