I would say that there are two years that could be called "the years the Linux desktop died".
The first one is the year 2011 - when Canonical decided to start "implementing their vision". I was one of the people that actually liked Unity, but in the end, whether it was the users being stubborn, or just the fact that Unity was/is an unstable piece of shit, it was what triggered the fragmentation of the Linux desktop.
The second one is the year 2013 - when Canonical announced Mir.
Regarding desktop development as a whole: I am currently working on a Windows application, and I can say with confidence: The desktop is dead - not as in "durr, everything cloud, web and mobile now", but as in "nobody is really working on improving it".
The problem is that the toolkits and the programming languages that can interface with them suck:
GTK - C API, very easy to shoot yourself in the foot with it, constant breakage due to API instability (or so I hear).
Qt - tied to C++, has bindings to other languages. Once again, building large software in C++ is not very productive and the alternative is using Python, which I consider the duck tape (type system pun intended) of Linux. Qt is definitely the best choice for native Linux applications at the moment, which isn't really saying much.
wxWidgets - Dead.
JavaFX - pretty much dead, and I wouldn't say that it was very good to begin with. The advantage is that you are building on the JVM, which has great languages and a huge amount of libraries. The disadvantage is JavaFX itself - it's like WPF, but nobody uses it.
SWT - still rocking, but relies on GTK/whatever and last time I looked its documentation left a lot to be desired.
Swing - dead.
Windows Presentation Foundation - Microsoft just doesn't care about it anymore, there are tons of bug reports that haven't been touched in ages and it feels like AngularJS but worse.
Winforms - pretty much dead. Not very flexible.
Universal Windows Platform - nobody gives a shit. Seriously, who fucking wants it? What would you even want to run on the Xbox?
Meanwhile, the JS ecosystem has multiple well-developed view libraries/frameworks, such as React, as well as languages (TypeScript, Elm) that make web development tolerable.