No one will ever make anything like a correct prediction about computing in 20 years time. Even 10 years time is extremely unlikely.

Would anyone in 1989 really have guessed that the majority of computing devices in 2009 would be mobile phones? Or that the majority of mobile phones would be sold in the developing world?

The only prediction I would make is that in 20 years time computing devices will be so cheap and ubiquitous that the hardware brand will be as trivial as the brand of pocket calculator you use. Most people won't care what the device OS is, as they will use their apps through online services.