It's not at all an essential control. I'd love to discuss this further, as to why.
Please do discuss. You assert it isn't essential but haven't said why. I know it was essential to me for many years. How will you prove it wasn't? Are you saying too few people care for it to matter? Or that the people who care are wrong to care and should be forced to adapt to your preference?
Am I saying that too few care to matter? Probably, to a degree. It's silly to think that majority of players use an inverted mouse for mouse purposes. Go to every computer in a computer store - all the computers there are going to mouse up when you push forward, and mouse down when you pull back. Look at any non-flight game out there. By default, you're going to look up when you push forward, and down when you pull back. Consider that the term 'inverted mouse' contains the word 'inverted' which means 'backwards vertical axis'.
Considering the camera movement in a first person, give the plane that the mouse is on the shortest rotation possible to become the same plane that the monitor is on. Pushing the mouse down at this point brings vision down. Pushing it up brings vision up. This perfectly logical.
Where inverted mouse (both X and Y) is used most today is typically when the origin of the perspective is not in the same point as the fulcrum of its pivot - such as a third person camera, or the nose of a space ship.
In the time you have spent denying it, it could have been implemented already.
And in the same amount of time you spent typing this reply, you could have downloaded AHK and pasted the code in.