By itself this isn't news (people come and go, it's a part of every industry), but when combined with other high profile departures, it doesn't paint a confident picture.
343, Ninja Theory, Tango Gameworks...
What was the last success any Microsoft studio has had?
This guy leaving before Hellblade comes out, Mikami leaving shortly after releasing HiFi Rush...
The biggest thing you have to worry about with big names leaving is that they'll take people with them over time. And what you want to see is a natural evolution where the next people up have been groomed for success to takeover. That's something they do at Naughty Dog for example, and what they're doing at Santa Monica.