I agree with you. All other things being equal, yes I would take more MPs. However, all things are often not equal. For example Nikon’s top of the line pro camera the D5 has only 20 megapixels. Several of their consumer and prosumer models have more. Some much more.
I do expect them to introduce their next pro model with a lot more pixels.
While I understand why you would call the D5 the best, I would not agree. Certainly it is the fastest and most expensive but is not nearly best for landscape and portraits. The D850 is a much better camera in most categories. I don’t think I would call the 800 series prosumer cameras. There are many more pro’s shooting the 800 line than the d3, D3s, the 4’s and 5 combined. The big reason for their popularity is their awesome ability to crop
at 75% and still have tack sharpness. The D5 could do a wedding but the 850, 810 or even the 800 would do a much better job. If resolution doesn’t mean anything then you might as well just shoot with a 700.