It would be good for Real Madrid if they didn't sell Casemiro in this season. His performance is good enough and he is still playing like a young. He has capability to serve few more seasons. Real Madrid will definitely miss Casemiro in their middle order.
I don't think that would be a good reason for Real Madrid when other teams are willing to pay Casemiro such a high price. Real Madrid have a midfielder who is younger than Casemiro, he is Tchouameni who has only been signed by Real Madrid this season. If Manchester United are willing to pay Real Madrid for Casemiro such a high price, then I don't think there is any reason to keep the 30-year-old again even if his performances are very satisfactory.