I'm an employee, and I doubt he's hiding anything. Even while working as CTO, he has kept an ear to the ground in engineering and periodically participated in design discussions and other things. I really think he just genuinely enjoys engineering work. Internally I think many of us have great confidence in our current leadership, so it makes sense that Mitchell would feel like he can hand the reigns over.

I mean, many great open source projects are still maintained by the original authors. Why would it be suspicious for Mitchell to move into a better position for doing the same kind of work?

Yes, in most cases I’d suspect some foul play. In this particular case though the cynicism is probably unwarranted.

He is fabulously wealthy now. He doesn't have to work. What is the percentage of his ownership in this $5B company? 10%? More? He's achieved his goals likely. Probably will want to slowly cash out and diversity.

Do other things...

> slowly cash out and diversity

I'm pretty sure that if that were the main reason, it could have already been achieved at one of the previous rounds.