I agree that tablets/smartphones won't fully replace PCs, but some use-cases are very well served by tablets, those mostly include media consumption (reading, videos, music) and "light" creation usecases (imagine taking a photo and uploading it, commenting on a blog, microblogging).
So for some use-cases, tablets can (and I think will, as more users purchase tablets) replace the PC, for others, they won't. Now if a user can replace her PC with a tablet depends entirely on wether the use-cases are narrow enough to not need a PC (e.g. no writing of long texts, coding, etc).
I've noticed this myself, since having a tablet, I do prefer this device to catch up on news, or play a game once in a while. For "real work", I definitely prefer a notebook or full workstation PC.