only siths deals in absolutes? Maybe try: "Unity didn't provide enough advantage compared to use an already baked DE compared to the cost of making it". I would be happy also with: "not enough people liked unity or mobile". Plenty of people liked unity and I'd like in this discussions a little bit of respect was given to the people who thought, designed and liked Unity. At least acknowledge their existence now that you "won"... I liked unity and actively dislike kde and gnome 3 (gnome 2 was ok but clearly nowadays is old and I still liked unity more).
Many people liked mobile too: I backed their failed indigogo and plenty of people did, but that was a weird move and it failed being TOO ambitious (I'm not sure they would have been able to deliver in hindsight).
When they did make a mobile, plenty of people would have loved to have a more privacy minded alternative to google (but still open), but they focussed on lower end where google was already present (and where this privacy power users are not) and, I think, made the mistake of discarding the option of going with android that had the apps needed to make the platform viable for users. In a way it's the "unity" move of mobile, but the difference is that on desktop you didn't throw away all linux desktop software.
I left linux on desktop a couple years ago, not because of unity but because
* I suddenly needed to buy a laptop, I wanted it with linux and the dell xps was about to be replaced with the newer. I also didn't bother to understand how to get a lenovo (possibly I wasn't aware you could get it with linux at the time, I don't remember) so I got a mac
* having a mac seemed like a good occasion to try some proprietary software I wanted to try for a while. Both the proprietary stuff and mac os were good and I could develop in virtualized linux so I couldn't see any reason to go back to linux for not linux.
So I think ubuntu failed in the desktop and mobile. Not that others did any better there in the linux camp.
Now for the armchair CEO moment:
If I had to go mobile AND drop unity, a few years ago, I would have rebuilt ubuntu as a multi form factor experience based on android, possibly replacing (or giving the option to opt out from) the google bits.
Now it could be late (but no problem I guess, as they are not doing it :D). This if convergence was the target, otherwise ubuntu + unity on desktop and android based ubuntu on mobile.