I'm really sorry to hear about this even though it's understandable that Magnus wants to spend more time with his family. It's good that he will still be involved in tennis with the tennisacademy.
We have seen how successful the working relationship between Robin and Magnus has been and I think Magnus didn't only helped him to become a better tennisplayer but was also good for Robin's personal development.
Hopefully Robin will find a suitable new coach with whom he can continue the great work of Magnus.
Good luck to both of them