I can think of a few reasons why they (or anyone) would want to (or should) ditch ubuntu.
Although Ubuntu is a glorified copy of Debian, the components that Ubuntu changes tend to be brittle and unstable, and not adequately justified. If anyone wants an OS that helps them get stuff done, not mess with their work, and not even break between major upgrades, Debian is the way to go.
Maybe another window has opened for some desktop innovation now? I would try to copy stuff from Android. Snap [0] is a promising way to deploy apps (in general, self-contained stuff you cannot depend on), even commercial/proprietary ones. The "app store" is terrible, though. Androids intents might be worthwhile to copy to decouple things. Androids activity lifecycle might be interesting. All of this requires a large-scale long-term reengineering of deeper layers, which is practically impossible for hobbyists. You will get hit a lot by people for being not-Unixy (see systemd) and only after years of slog you might be able to show a superior desktop.
On the other hand, there is no money to be made with desktops, so why should a company approach such a risky project?
One of those examples where competition does not help at all.
Many desktop users and contributors moved to Ubuntu and stopped contributing to Debian. A good number slowly moved back to Debian in the last 5 years.
There is a similar situation with clang and gcc. Clang improved error messages and raised the bar. Gcc follows. It would probably not have improved on its own.
[0] https://arstechnica.com/information-technology/2009/06/canon...
Then of course companies like Google are immune to this sort of issue as they even design and sell linux hardware.
Even on the Asus EEE 1215B that I bought with Linux pre-installed, I had issues with my wlan card.
Spectre patched kernels were only just released into -proposed in the last day or so.
Ubuntu Meltdown: Jan 10 Spectre: Jan 11
Using debian could maybe be compared to using vanilla JS instead of the shiny frameworks.
The downside is that, debian is very conservative in their releases. Most packages that ship with the distro or are in the repositories are a bit behind. I guess that's why Google switched to Debian Testing.
Btw even debian testing is comfortably stable.
For people who want to try debian, I advise them to try Linux Mint Debian Edition. It is a vanilla debian shipping with a good looking DE. I installed it on my mother's laptop, she's been using it for more than 2 years. It is so stable that, one time her laptop broke and we pulled her old netbook from storage. I only took the hdd from the old laptop and inserted in the netbook. After installing the wifi driver the machine has been running flawlessly for more than a year now.
I only do dist-update dist-upgrade from time to time, no further maintance was necessary.
Actually even recently I got email that a bug that I was watching was closed because it was too old. I mean I stopped carrying about Ubuntu almost 10 years ago.
The vast majority of issues I was having were around desktop functionality: connecting a monitor to laptop, weird issues with UI, wifi, putting laptop to sleep. Actually one of last issues that infuriated me was them being ok with release that that crashed (froze) every time you started a laptop with a proximity to 802.11n network.
Imagine trying to use your laptop to take notes in college and it froze few seconds after it booted up repeatedly. It took me a while to find out what the problem really was, then found the bug, realize that it was opened before the release, yet they still went with the release.
To make things even more frustrating at the same time they refused to include OpenOffice 3, because it might not be stable enough. Talk about priorities.
That was the time I started looking at alternatives, someone recommended OpenSuSE and I really liked it.