Add to this the notion that effective virtualization is becoming more and more realistic via a cloud service, and we will likely end up with tablets/smartphones as the predominant “personal” computing device, and possibly what amounts to almost a dumb terminal on our desktops, for those occasions when we want additional screen real estate and a desk-top environment.
Additionally any of those three device categories could access a powerful-but-cloud-based virtual machine which will, in the very near future, possess most if not all of the performance characteristics of our current desktops. The only bottleneck at that point will be network throughput.