the default desktop

Debian doesn't have a fixed default - but it happens that GNOME is at the top of the list in tasksel. Many of the smaller lighter weight desktops eventually end up pulling in some component or other from a larger desktop at some point as you add your favourite program.

It's not a zero sum game: in the long term, no desktop "wins" as there's enough space for several desktops, many use cases.
The caveat is that there aren't enough developers to go round - many attractive desktops in smaller distributions which draw people to them are, in fact, a reskinning of something like Ubuntu as the upstream. My colleagues who say "I'd like to get into Linux but there are hundreds of different Linuxes" miss this point and stay on Windows because it's more consistent for them.

What does make it difficult is when a major vendor has more or less abandoned the workstation because they focus on servers or cloud components. That just means that everyone uses Windows by default in that environment.