Sales of traditonal PCs with software compared to gadgets with apps is going to yo-yo for quite some time yet.
For a technology to be compelety replaced, the replacement can't have huge negatives. The negatives to mbile OSes and apps are huge.
Performance of mobile gadgets and apps will improve of course, but the very basic nature of those the negatives of mobile vs. PC are not.
Phones and laptops break very easy and its very easy to for them to get lost or stolen. You can't rely on them to solely store any important information. When flash memory dies or when your ipad gets dropped your not getting your data back.
This is also a big reason why SSDs haven't replaced mechanical drives yet, not just size/price like everyone assumes. When SSDs completely die, and they do, your data is history. When old drives die, you can more often than not find someone that can get it back. I know I can most of the time, and when I can't, data labs can. There not much a lab can do for an exploded flash chip from battery/PSU power surges.
You can't count on people backing their stuff up, because most just do it. Trust me, I run a computer store. Sure you can keep your data online, but there's lots of data people want to keep but don't want it on the internet. That might take all day, or ruin their privacy and lives if that data ever leaked out.
Clouds are getting popular, but as soon as a story hits the news that someone's "special" photos of his wife leaked onto Facebook from a hacked LiveDrive, people will look back to the trusty old PC. Hell my Hotmail just got hacked by someone who sent out links to Chinese e-tailers and they tried to makes changes to my PayPal account. Think I still trust Hotmail?
Also, the first time people lose everything they had on their smartphone/tablet they will realise they just can't do away with PCs.
Texting, mobile Facebook, mobile photos and casual app popularity are totally skewing sales assumptions. Millions of phones are selling just for these purposes alone.
This is leading "experts" to assume smartphones are suitable PC replacements. If you are going to assume that, you might as well wrongly assume smartphones will kill digital cameras and photoshop too.
This is not a changing of an era. This a new additional and alternative market trend.