There has been much debate over the years and it’s easy to see there is a lot of momentum and gravitation towards smart phones and tablets because they are convenient and are capable of many things a PC is.
The stats show the following trends in my opinion: http://www.statista.com/statistics/272595/global-shipments-forecast-for-tablets-laptops-and-desktop-pcs/
The laptop market peaked in 2011 at 209 million units and is projected to be 170.4 million in 2019.
The laptop PC market peaked on or before 2010 at 201 million units and is projected to be 170.4million in 2019.
Tablets have had amazing growth from 19 million units in 2010 to 269.4 million projected in 2019 with a peak of 407million projected in 2017.
The numbers of PCs and Desktops show some variation but are not an immediate indication that people will stop buying them. Certainly the tablet market has probably eaten into some of the laptop market. If anything the largest drop or decline is being forecast for the tablet market. I think part of the reason sales are slowing in the PC market is not because people are not using them, but quite honestly even old computers are capable of doing most tasks people want them to so there may be no need to upgrade.
With that said, I believe most professionals will still prefer a PC for many uses. Personally speaking I can get a lot more done, more quickly with a PC. I also feel PC’s are more secure than smart phones because with the phones, we never know what is in the ROMs or what potential security flaws are waiting for us.
When using a Unix based open-source PC there is reasonable assurance to privacy and security compared to a tablet.
The server side is something that is not only, not declining but growing and ironically this demand is being fed by a lot of smartphone users putting their data into Cloud based services. Datacenter services have not fundamentally changed in a long time, there have been experiments in cooling and green options but whatever the improvements and format, server based computing is here to stay long into the future.
PC based computing does appear to be on the decline and may continue to decline as more and more new generations grow up almost exclusively on an iOS or Android device. However I do not think it will ever disappear in the foreseeable future and the server side has a long way to go.