The PC has been available to consumers for over 2 decades now. I think it is amazing that growth was sustained for as long as it was.
I highly doubt the tablet curve will enjoy the same kind of sustained growth. It will peak and it will level off. My '09 2Ghz Core 2 Duo does everything I need it to except for gaming. The Samsung 830 SSD I put in made it faster for day to day use than the 2.4Ghz i5 I was issued with a regular HDD.
My parents are using the PC I built back in 2005 and the computing power vastly outstrips their needs.
I simply cannot justify purchasing a new computer because it would be consuming for the sake of consuming.
I think smartphones have already reached the point of diminishing returns. I progressed from a BB Curve, OG Droid, Thunderbolt (utter POS, ruined HTC forever in my eyes), and my SGS3. There was a clear and tangible increase in performance between each phone..well, the HTC had the performance increase but I would be at 60% after a couple phone calls.
I have zero desire to upgrade my SGS3 to a SGS4 or HTC One or whatever wizbang phone out right now because it does everything I need it to without slowing down. I suspect if phones were not subsidized as it were in the states, sales would not be as robust. The case may be a little different for phones since it sees more daily wear and tear but I don't think Apple or Samsung will be able to maintain their spectacular numbers anywhere near the extent that PCs did.
PCs have simply hit the decline stage of the lifecycle curve as a whole. I don't think tablets will crash and burn like the netbook market did but as of now, it is complementing the PC. I doubt an appreciable portion of tablet owners have it as their sole computing device. Given a choice between one or the other, they would choose the PC the vast majority of the time.
If my tablet breaks, I can wait to replace it, maybe wait around for the Nexus II to come around instead of replacing it with the current generation.
My laptop breaks? It is getting replaced uno pronto. I don't even care if Haswell is right around the corner, it is getting replaced.