I don't have a problem with middle click being a backwards-compatible convention. It's that the option to change it is non-obvious, and therefore excludes laptop users.

As a counter-example: Much earlier versions of Windows had a menu in the top left corner. The shortcut to close a window used to be double-clicking in that corner. In Windows 7 (not sure about 8 or 10), that menu button/icon is gone. Of course, you can close a windows with the X in the right corner, but the old shortcut still works even though there is seemingly nothing there.

I can't remember where I read this, but I always think of it when it comes to UX design:

Always have more than one way to do the same thing in a UI. Users will pick whichever works better for their worklow.

I wasn't defending it. I'm familiar with it and use it occasionally, but I don't think I would have made that design choice.

hmm, horses for courses. I thought popular thinking had moved on from the perl philosophy of "there's more than one way to do it" to "let's reduce complexity and errors by just having one way to do it". While having a CLI and GUI might be good for two different UIs what's the point of having two ways to do the same thing?

The windows X is more of a backwards compatibility thing I imagine, because it makes no sense to have an invisible menu in the top left corner.

There are two ways of doing copy/paste. The middle clicking and Ctrl+C/V.