The point is this: "It is much easier to improve mobile platforms enough to replace PCs than it is to try and simplify PCs and make them more suitable for mobile usage scenarios."PCs aren't going anywhere. They will always have massive advantage in speed, storage space and obviously display size.
However if Google decides to make Android as free OS for PCs... Things would get interesting (assuming that they improve power user keyboard+mouse experience a lot).
Both Android and iOS are working on making multiwindow support. Google is also supposedly integrating Chrome OS to Android. With Office for iPad and multiple tabbed browsing on iPad Safari/Chrome/etc, people like my parents could stop using their computers completely. (I still like the Surface Pro/Book better though.) It's not going to be too long where the most consumers could drop the PC.