So then the question is - why have a separate OS for tablets as compared to laptops? The current reason is not so much to do with the UI but more to do with management of limited resources. OS's like iOS and Android are very aggressive about preserving memory and processor cycles as a way of squeezing extra life out of batteries.
On phones that different-OS resource advantage will probably continue. But on tablets, its less clear. As the intel chips get better at low power, a tablet running a 'full fat' operating system becomes more feasible.
This is what the Surface Pro 3 shows, whatever you think of it as a device: You _can_ run a full OS on a powerful processor in a fairly thin, fairly light tablet. Not the thinnest, and not the lightest, but still it raises the question: Why do tablets need a different OS?
Microsoft are tackling that question now, I think Apple will ultimately need to tackle it as well.