I have been discussing something like this with some friends recently.
I have no doubt what-so-ever that Android will be around and be a Big thing.
But, in 5 years, mobile processors will be as powerful as current laptop processors and maybe even desktop processors.
Given that, will mobile operating systems even be needed any more?
OSX may show up on a tablet with this "iPad Pro" rumor. or maybe a hybrid OS of OSX and iOS.
Microsoft keeps making windows "lighter and lighter" so as of Windows 10, any app made will run on anything from the phone to a desktop.
My Question would be, what will happen to Android. I have no doubt they will still be here, but of the 3 phone operating systems, android is the only one that doesn't have a full desktop version that can also run x86 level programs. I don't see a phone being able to run Final Cut Pro or the like, even 5 years from now.
Android tablet support is still not that great. Most android tablets just run phone apps blown up. Some developers are remaking apps for tablets to make the experience better but not nearly as many as iOS developers. I personally wouldn't even try an android tablet for this very reason. To be fair, I don't own an iPad either, but if I were to buy a tablet, it would be iPad or Surface Pro 3 (no RT garbage).
Anyways, Good question. With cloud computing being the future, android may be the only OS needed. Maybe we won't even need "desktop" operating systems. Who knows. As of right now, a chromebook isn't a computer and could never replace my desktop.