To me the easiest way to determine if Linux desktop has missed its chance or whether it ever had a chance, is to look at what we value about Linux Desktop, and how that relates to everybody else.
A non exhaustive list for me:
Stable software with a low memory footprint.
I never feel this way with software on Windows (haven't used Mac's in earnest), they always feel a little extra bloated. My linux desktop (MATE with firefox/terminal) always feels snappy, and responsive.
Does everybody else care about this? In general, I think yes. The amount of times my wife turns on her computer and complains that program X takes 20 seconds to load. Is it a definitive enough feature to see a significant chunk of people re-install their OS? It is not.
Two things here, I love the fact that I can go to one place and get my software, secondly I love that updates come through the one system, and that program X, Y, Z are not all updating at different times with completely different interfaces to this update. Of course there are exceptions to this, but they generally come from programs like google chrome or the likes.
Does everybody else care about this? I would think to the first part (having one place to install software), no. For Desktop I think people still are not sold on the app store thing, but they potentially could be. For this to be a killer feature for people the repository would have to be jam packed with more apps. With the proliferation of apps on the web, I don't see this as a big deal, once people have Chrome or Firefox, they'll be mostly happy.
On the second point with regard to a single place for updates, not sure people care about it, but they would enjoy it if they had it. Its often a source of complaint that program X (usually windows itself) has gone down for updates. Having that controllable through one interface is great.
Again, not a killer feature for people to mass flock.
Open Source Software
Again, multiple reasons, one is the ideals behind it. The fact that for the majority of the software I use, I can fix things, add things as well as interacting with the core maintainers. I value this, but rarely use it, especially with big projects like Firefox etc.
The fact that the Linux has open source at the heart, I think it breeds certain other advantages. No adware in free software. Software which is not constantly reporting home or spying on us.
Does everybody else care about this? No. As much as someone of us do, the vast majority of people have no interest in being able to see the source of their software. They also for the most part don't want to be involved in the community of their text editor.
Much like the show recently discussed when how a third player would break into the mobile space, they will need a killer feature that will bring the masses. Desktop Linux has many things which would make you not want to move from Desktop Linux to X other operating system, but that is only for us who have been bitten by the Linux bug. And the things that bit me, have no affect on the vast majority of other people.