There are now closing in to 4B Smartphone users, and about 1B Tablet users.
PC usage isn't really shrinking, Office, Gaming and Productivity are still holding up. But most "consumer" usage has definitely shifted to Post PC.
Years later, the same kind of slowdown happened with tablets (especially Android tablets which are basically dead, but iPad wasn't completely spared either), and will eventually happen with smartphones as well.
The "we are now in a post-PC era" hype of 2010 (after the iPad's release) has eventually led to many "tablets are dead" articles by 2016.
I don't know how long I'll be able to run that server for before needing to upgrade something, but I suspect its lifetime is until something catastrophically fails.
For Web browsing and office work it works flawlessly. One day I'll throw in a cheap SATA SSD that I'll find on sale as the OS drive and it'll be good for even more years.
The only exception that I can come up with right away is drawing, but even taking pictures, well my phone doesn't hold a candle to my DLSR.