John Sheehy wrote in post #18418311
10mm on APS-C1.6 gives the FOV of 16mm on a FF. APS-C also crops the image circle of fish-eye lenses, losing the rounding edges, unless there are APS-C-designed fish-eyes that I am not aware of.
While I see no intrinsic value to a crop for putting more pixels on subject (pixel density is what matters there), the wide end of the FOV spectrum is substantially served by a larger sensor, until the day when all FF wide lenses are replicated for APS-C.
A couple of points about your second paragraph.
1.) My 80D puts more pixels on target than does my 6D, with the same framing. If I need to crop, the 80D will give me better results. If I don't need to crop, the 6D will give me better results.
2.) I don't EVER see the spectrum of EF-s lenses even approaching the breadth of the spectrum of EF lenses available, especially at the wide end. Who would pay big money for an EFs15mm f/2, when you'd get better DOF control, out of a (likely) less expensive 24A. Oh, sure, I could buy an 85II, for my 80D. But the much cheaper 135L does the same thing on my 6D for less than 1/2 the money. And the 85 1.8 works just fine on my 80D.