[fock]

I don't think the people who are non-obvious rich put their money solely in an index fund. They'll generally have about 50-500 apartment units in some big city, which (if properly insured) guarantees you can weather anything except for wars and asteroids. Maybe they put their earnings into an index fund though ...

[lacker]

Not solely, but index funds are probably the most common investment. Just look at the total money Vanguard has under management.

[lonelappde]

$5M in Vanguard also is safe from everything but wars and asteroids.