I actually disagree on some of this
the main thing I have against Casemiro is that he is a thug who gets away with murder
but he is superb positionally, and is not that slow (not a burner, but generally fast enough, although he is slowing a bit as he ages)
he plays almost as sweeper at times, he is vital for their defensive play
you are correct in terms of his not so stellar passing at times, and yes he is hardly a control genius
but he is so not a weak link IMHO