Development has slowed
I do not know that much about the general desktop computer sales but I imagine the main reasons for slowing of sales are:
1. The technology for new systems is not growing as fast as in recent years. I am retired and consider myself an enthusiast. I used to spend a thousands of dollars every year or two to keep my system current or near current. My current system is 2.5 years old. The only recent update as been to switch from using two NVidia 760 GPUs to two NVidia 980Ti. My 3 year old processor is still runs at about 90% of the processing power as a current Intel i7 5930 processor and only slightly slower than the current i7 170 generation processors.
2. As previously indicated, new processors are not providing significant increases in processing power. While workstations can be replaced with systems that have the lastest instruction set and more processors, the software has not been modified to take advantage and automatically scale to the number of virtual processors available to the system. Most programs still only reside in one processor at a time. Until programs useful to the specific businesses are written to take advantage of the latest instructions or designed to scale with number of processors, most businesses will see at best a 3% increase in processor speed. This increase is generally not sufficient to even be noticed by the individual users as most are using very little of the processors they currently have. The increased speed is therefore useless to the business world and there are only so many enthusiasts out there.
3. There have been no significant increases in storage technology. Yes, Seagate has come out with their 8TB and 10TB drives but those are really only useful for long term archival. They are not suitable for constant reading, modifying, and writing. Further the speed of the drives has not significantly increased even in the SSD category with the only real exception being drives such as the Intel DC P3500 series which plug into a PCI-e slot and the m.2 SATA III drives.
4. Memory speed increases provided by DDR4 compared to DDR3 are again not noticeable by most users outside the enthusiast community. Unless the user is processing a lot of data or doing CPU/memory intensive processes, like video editing, the speeds gained by the new memory, I would guess might save the 30-60 seconds in a day. Again not enough for the user to notice.
5. No new technologies have appeared in the last year that call that call to businesses or to enthusiasts. The only real changes have been in software such as Windows 10, which will almost surely be an anathema to extremists that like full control over their system, and cloud processing and storage that actually reducing the processing requirements of workstations and home systems.
To sum this all up, at present there is little to no reason for businesses to upgrade the workstations in the individual work areas in the current machine is less than 3 years old and probably less than 5 years old. Private users fall into the same category. Desktop systems are not on the way out. Businesses must just scale to reduced change over as systems now have a much longer life/duty cycle before replacement will be required. It is much like automobiles, once everyone has a good car they only replace when it starts having problems or they feel it is just time to replace their old but still functioning system.