Fifty years ago, some people said how we would have fusion power and ArtificialIntelligence within fifty years. Nowadays, some people seem to say the same thing. And add to it MolecularNanoTechnology. What should we believe?

Only two things are certain. One, the world will not continue as it currently does. Two, the changes in store for us will certainly and dramatically affect our personal lives.

''WithinFiftyYears, humanity may have found a way to destroy itself... at least one pundit has stated a 50% chance of a mankind-caused CatastrophicEvent which kills scores of millions and threatens civilization, occurring in the TwentyFirstCentury.''

''Certainly there are people with the means today, and people with the desire.  Fortunately, the two sets are currently disjoint--as far as we know.  But for how long?''

I think you're referring to the "Doomsday Argument" by Dr. Nick Bostrom.
http://anthropic-principle.com/

See also
WithinTwentyYears,
http://futures.wiki.taoriver.net/
,
TheDoomsayer

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In Fifty years, many people reading this page will no longer be able to do anything they do today, either because they are dead, or because the world will have changed so much that no one will care to do those things. 

Those who were alive in 1957 and have survived until today can testify to the magnitude of change being "Un-imaginable".

''Has it really though? We still watch moving pictures on rectangular flat surfaces, still move around controlling horseless carriages by means of a vertical wheel, still shovel food into our mouths using metal utensils... Some of the predictions from 1957 (well, 1962 as I write this) are what's unimaginable. 3d television, atomic powered cars controlled by a joystick, food pills. Self-driving cars were also predicted, but those don't seem too far-fetched anymore.''