In the 70's the market was sideways. In ten years it advanced only 9 points.
So while the 50 year span shows impressive markets, each decade has had its downside.
During the fifty years a lot of things can happen. For example a guy in Poland who invested his money in 1910, could experience a few quite impressive downsides before getting to 1960. World is getting smaller, changes are accelerating, who knows what is in the future?
Things can go unexepected way on the individual scale too.
The ground of a certain rich man brought forth plentifully:
And he thought within himself, saying, What shall I do, because I have no room where to bestow my fruits?
And he said, This will I do: I will pull down my barns, and build greater; and there will I bestow all my fruits and my goods.
And I will say to my soul, Soul, thou hast much goods laid up for many years; take thine ease, eat, drink, and be merry.
But God said unto him, Thou fool, this night thy soul shall be required of thee: then whose shall those things be, which thou hast provided?
(St. Luke:12:16-20)