Indeed some dumbed-down "desktop GUI", forced upon everyone in the name of "challenging" a monopolist, would be a stupid move, just making the OS less attractive for those that are not technically challenged and able to make their own decisions on what to use.
Windows never needed to compete with anyone on PCs, and especially not by trying to be attractive for users, because the majority of users was always forced to use Windows by some employer. And those not particularly interested in technology for their personal purposes used whatever came shipped on their latest computer.
The idea that the looks of one specific desktop GUI is what determines "success" is stupid - just look at how often both Windows and MacOS changed their looks - back and forth - and people certainly did not change to a different operating system in masses because of that.
Linux should sell itself by its most obvious strengths: Being a tool for the owner of the computer, not making the user a tool for the OS vendor. No unwanted ads, no data harvesting, no unwelcome obsolescence of still good hardware, no walled-garden "app stores", that is what can really separate open source operating systems from the dystopian paid subscription, closed source, vendor-lock-in commercial operating systems of today.