Why leave?
—> 16 years in not simply “start up mode” stress, but “Elon Musk start up mode” stress. The last 6 years of this daily tension while knowing you, and generations to come, are ridiculously wealthy.
—> Since I’ve been following the company closely (2012), perhaps basically all of Tesla’s history (even while others were CEO) it had been very clear that JB was Elon’s “number 1,” so to speak. That is, his most trusted and valued contributor and companion on Tesla’s journey. Then Jerome was made head of automotive last year. What’s more autonomy has clearly by far become the most captivating area of development for Elon, and he seems quite taken with its driver, Andrej Karpathy. (gratuitous speculation, but I wouldn’t be surprised if Musk looks at Karpathy and thinks, “hmm... there may be someone with a more capable mind than myself... and must be nice to be so consistently “chill” yet energetically positive”).
It seems JB has basically gone from sort of Elon’s little brother, always by his side (literally brought on global trips and at his side 2013-15) and presumed heir apparent when Elon leaves CEO role for more SpaceX time (which once was a strong possibility once the Model 3 hit volume production, but not too likely for years with the CEO agreement Elon made with Tesla a year or two ago)... JB seems to have gone from that to part of the second tier orbit around Elon’s focus, barely noticed amid all the energy around day-to-day drama for which Jerome is his key player, and Tesla’s perceived FSD future, led by Karpathy.
—> Elon and JB seem to have such different personalities. Somewhat speculative, but I suspect Elon’s behavior towards people grates to some extent with JB. Given the first two points above, it’s probably become harder to ignore. The intensity of the last two years and rough treatment/letting go of large groups of employees have probably also made it harder to ignore.