Put him beside Mainoo (or someone like Mainoo - i.e. an actual central midfielder with legs who can retain the football) and he'll look a lot better.
We have an ageing midfielder whose recovery pace isn't particularly great and who isn't particularly careful on the ball and whenever we play him it's been with two number no.10s.
I think Casemiro is clearly on the way down, but we can make that a very gradual decline instead of a steep fall if we actually make reasonable adjustments for his limitations.
What is severely lacking in these thread is the acknowledgement that many exceptional players also change their style of play and figure out how to work around ageing. Typically, players have 3 phases to a career: excitable youth; peak years; and the wind down before retirement, there's very few who exit the game how they entered or peaked in it and that doesn't mean they nosedived at the end as most have the self-awareness to get out whilst they are on top.
Even if Casemiro were on a certain slide, there's nothing to say he wouldn't learn how to work around it, where this thread is mostly binary 1's and 0's, which is not how things go for the majority of players.
First and foremost, not having a stupid structure in midfield is beneficial to everyone, not just Casemiro. If we want to play with two #10's they have to be hybrid #8's who are extremely comfortable in midfield and being connective deeper and after that, having the backline who support the #6 and the inversion besides that who slot in to cover spaces down outer-middle of the pitch. None of that was in evidence when Casemiro had to run about doing too much work and abandoning all the principles by which he made his name. It's actually crazy that the majority don't factor that into the assessment of why this once composed and calculated midfielder was suddenly looking rather amateurish and shell-shocked with acres of space all around him and zero support.
A solid system and reading of play is a massive factor in controlling midfield; these days, the lose men who break free of that minefield to run at a backline are isolated and far away from goal, faced with pacey CB's and inverted fullbacks who will immediately pressure them. Until we have that, blaming a #6 makes little sense.
As you've mentioned, Casemiro-Mainoo is the pairing I think many are eager to see, but I wonder whether that two #10's will be put to pasture and a double pivot utilised or even an #6, #8, #10 set up. I'd expect a massive upgrade in a midfield consisting of the aforementioned two plus someone ahead of them. In Mainoo, Casemiro would finally have a worthy partner by which he could be properly assessed.