You don't judge a 30 year old signed for massive money on a 4 year contract on his first 6 months.
The problem with the Casemiro signing is the risk that his level is going to drop, not his ability currently. He's 31 in two months and he still plays in an all action style which relies on his physicality as you saw by his assist, it doesn't take losing too much pace for that challenge he makes to be a foul or for him to miss entirely and be caught on the counter.
If he plays like this for 4 years it's not a problem, but what if this time next year he starts to see the same decline someone like Matic did while he was here? 18 months of even world class performances followed by 2 and a half years of being unreliable for £60m and £350k a week would be a problem.