Casemiro and Varane left for the same reason, which was that they had enough with all the achievements at Real Madrid. Incidentally MU offered a higher salary so why not. Winning 5 UCL for Casemiro is indeed an extraordinary achievement, especially since he has won all the trophies at Real Madrid.
It's time for Casemiro to consider the big salary issue at Manchester United because his career has also been very bright at Real Madrid and also has extraordinary achievements as you say. Hopefully Casemiro can be very comfortable at Manchester United and still be able to give everything for another great achievement at Manchester United because he is not that old either.
He may not be that great but he is not the type of player that can turn a team into a winning team. They wanted to get De Jong and couldn't afford him, now they are looking at Casemiro, but they should have gotten a look at defenders instead, they think all their problems are about midfield and that’s a big mistake, looking at the 4 goals in 35 minutes, that wasn't about the midfield at all, that was about defence.
In the end, this transfer could happen or may fail and it would not change anything for United, whatever they can do with him, they can do without him and the result will be the same not because he is bad, but United is so bad that he can't change a thing.