I think the personal computer will split into two branches, with power users keeping today's form factors, while average users who just want to surf the web or check their email will go for the appliances.
I think you mean: evolution doesn't always mean _adding things_. Taking away something that causes a net loss is evolution. Much like, say, the tail in certain higher primates. Or the ability to install and run absolutely anything (including malware, trojans, and viruses).
(Whether a thing causes a net gain or loss is its own entire argument, of course.)