Can someone ELI5 why this even matters/is such a big deal? Does the default DE have its tentacles so deep in the distro that it can't be changed by users to suit their preferences?
I run i3 on Debian, and...well, actually, there is no "and," I just installed the WM I wanted and that was it. And as I recall the installer asked what DE/WM I wanted to install anyway.
Its a huge deal. If X desktop is the default, it shows that the distro developers and maintainers usually test and optimize more and better for that specific DE so your experience with the default DE will always be more stable and polished than non-official ones. Extra GUI tools that the distro makes usually are also better tailored to the default distro. Like Manjaro and all of their locale, kernel and other packages that are integrated inside the KDE settings. Or popOS and all of their utilities being integrated into Cosmic. Etc etc. More money and dev time is invested into the default DE.