Berkshire AGM 2017 - Why Warren's wife Astrid will get an index fund (not BRK shares)

Warren Buffett has already distributed about 40% of his Berkshire shares to philanthropic causes, and on his death, it will all go to charities. His wife Astrid will only own the Vanguard S&P 500 index tracker. Buffett was asked why he has chosen the index fund over Berkshire shares - this segment of the AGM starts with his answer...