> But here you are already assuming that all these developers want to have a modular desktop environment
Those that don't want a modular environment just use the defaults someone else has created.
Modular doesn't mean you are forced to customize. You can still just choose/use your desktop setup as a single decision.
But here you are already assuming that all these developers want to have a modular desktop environment. Not everyone wants that, or workspaces, or docks or whatever.
So yeah, if everyone wanted the same thing, I guess we could end up with one desktop environment that covers everything everyone wants. But (fortunately), the world is more complicated than that :)