I don't agree that W10 is like an OS "with two mixed worlds".
Microsoft is trying to provide users with a more consistent working experience across devices (desktop, laptop, tablet, phones). Such a consistency is very important. As users, we are tired of searching for apps across devices, for just doing the same task. We are also tired of getting our devices into the same settings.
If your desktop OS and mobile OS come from the same company, for what reason would you like to work on two inconsistent UIs? We can accept that the mobile OS has less functionalities, but we expect the same UI experience.
As smartphones are becoming more and more powerful, and mobile network is getting faster and faster, we should all agree that smartphone has become an vital device for working/playing (e-mails, calendars, documents, music, ...). A consistent UI is therefore of utmost importance to users.
If you say different devices are of "different worlds", then you are wasting your time on learning how to do the same thing on "different worlds". As users, the primary focus is on working & playing, but not on learning how to use an OS. A good OS takes users minimal time on learning.
Those people making unnecessarily too much criticism on Win8/10 most probably just hate Microsoft.
It is a very old mindset to think that desktop computers and mobile devices are two different "worlds". There would never be smartphones if we think that browsing on the Internet could only happen in a desktop computer. There would be no tablets if we think that watching movies could only happen in a desktop computer.