I am an ambidextrous mouser, mainly because early in my computing career I found mousing on the right hand was leading to RSI type pain. I started using a left mouse as well and the problem went away. Now all my machines have two mouses (apologies if I am using an incorrect plural).
The issue is with where the select and menu click is on a mouse. If you use a right handed context then your index finger is on 'select' and your middle finger is on 'menu'. If you are a left handed user you have to switch the mouse config over to get the same functionality. However when you do this the buttons on the other side become transposed which also elegantly demonstrates why the term 'left click' and 'right click' are an oversimplification.
So far X11 is the *only* windowing gui I have found that supports ambidextrous users so that the buttons are configured correctly.
The other, more subtle use case is the cut and paste paradigm. In X11 I select text boundaries with a 'index' and/or 'menu' click, then paste with a middle click - my hand doesn't leave the mouse. This is a massive boost of productivity over time that I think is an undervalued feature of X11, surely I can't be the only one who appreciates it?
It's a fairly popular use case to run X11 on my local display to support (servers/clients) over ssh. I have only tested this indirectly (on a colleagues machine who needed it) however Windows 10 seems to have killed Cygwin running X11, which means it is now more difficult to run X11 on Windows.
It would seem to me that the mass of user demands is overwhelming the power and more esoteric use cases that X11 satisfies.