In 20 years time, just as it was 20 years ago, power will still be proportional to size.

Yes, tablet devices in 20 years time will be vastly superior to the best PC's today but something that fits in a PC sized enclosure will still be that much faster and more powerful again and, hopefully, programs and applications will be pushing those machines just as todays applications push todays machines.

Will PC sized devices still be around in 20 years time? I hope so...