Simply dropping a blade that hard could shatter it, I would just as soon sharpen 60-62 rockwell blade more often than chance the brittleness of a harder blade. To each their own.
I don't know that much about CPM M4 but steel can be very hard and not be brittle. H Roselli produces a modern Wootz steel with a hardness of 66-68 HRC without being brittle at all. You are correct though that many steels will be very fragile at such hardness. I just wanted you to know that they don't all have to be.
