How do I know this? Go to a developers' or tech conference, and what is the prevailing OS? 9/10 times these days, it's MacOS.
As much as Apple may be control freaks, they did desktop UNIX right and developers have flocked to them.
Meanwhile, the Linux desktop community is still having the same debates, the same difficulties it had back in the mid-90's. Dependency issues. Lack of compelling use case software; and that which does exist is versions behind current. Fragmentation.
The only areas in which the Linux desktop has moved forward is UI and user-oriented management, and Ubuntu deserves much of the credit for the latter.
Add to this the gradual movement away from the desktop paradigm to mobile. The desktop paradigm will still be around for several years (especially for creators--developers, media folks, etc), but the end-user is moving more and more towards smartphones and tablets for consumption. Extend those with external keyboards and monitors (think docks) for light creation work (documents, spreadsheets, etc), and you've the future.
Nope. The Linux desktop is dead.