He's been at Microsoft for 23 years and it's basically the only company he's ever worked at. He's also over 60. I can't blame him for wanting to try something else with the tail end of his career.

SPJ had a great deal of latitude at MSR Cambridge, he could probably work on almost whatever he wanted. The problem is MSFT is rife with bureaucracy and it is a constant fight to get anything done. MSFT in general and MSR Cambridge specifically has lost a lot of their best people over the last 10 years. AFAIK it is expected to get worse under Christopher Bishop. With the recent win in Excel Lambda it is as probably a good time as any to get out.

This is precisely why SPJ leaving at this time is all the more surprising.

(I left MS recently)

Eh, I don't think it's any different than the general trend of people moving around. 23 years at a company would indicate that it's been a pretty solid place to work.

At the same time, leaving after 23 years for a reason other than retirement suggests something changed.

I have no insight into the company as you do, but that is the first impression I’d get from the circumstances. People don’t stay at a company for 23 years only to jump ship for the same reasons early-career people jump around.

Or you realize that you've spent basically your entire professional career at one place and this is your last chance to try something different.

The entire industry is experiencing an unusually high Turnover.

Bureaucracy.