I think his legs
are going rather than totally
gone already -- but the things you mention above are exacerbating it. At least, bullet points 1, 3, and 4.
I think it's mostly the fact that he is:
1. a 31 year old defensive midfielder who is mostly having to play behind two slight no.10s. Mount and Fernandes might press and cover distance a lot but they're both non-entities for defensive work when compared to actual central midfielders. Eriksen is even worse.
2. What
@Brwned said on page 33 and what your third and fourth bullet point allude to.
Casemiro is at the stage of his career where he's done all the water carrying at an elite level to win all the silverware he can. He's now just out there vibing; sometimes defending, sometimes having bizarre runs where he's the furthest forward, sometimes crazy fouls, sometimes bagging 20 G/A's in 60 matches.
If we put him in a disciplined no.6 role with a disciplined balanced midfield around him, we could definitely preserve his shelf life for a little longer.
I don't think we, nor him, actually will do that though. A reckless blaze of a final season this year before a move next summer seems likely in my mind.