[Brightcolours]

You make me chuckle. Now give a similar "rant" about the focal length number. And then realize that indeed the focal length and f-number stay the same no matter which size sensor one puts in the camera, and that it was about equivalents. 

Equivalent focal lengths give a similar FOV. Equivalent f-values give a similar DOF (and similar apparent aperture, which lets through a similar amount of light).