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.
