Proof of Stake means that the greater "stake" a person has (i.e. the greater number of coins), the greater chance of forging the next block and reaping the transaction fees.  There are 1 billion NXT and if you own one, you have one chance in one billion to forge the next block.  It's not exactly done like that but the result is about the same.  Also, if you have a large stake and try to force a fork by not forging a block when you get elected to do so, your forging power drops to zero temporarily which allows others to pickup the slack.  Again, I'm a novice at the exact mechanisms of NXT forging.

To address the possibility of "living off the interest" of your fortune I say…. it's a free country.  Eventually you will die and pass your coins to your heirs and they will spend them, distributing that wealth to the rest of society.  The same thing happens in any capitalist society based on morality.  (no forced distribution of wealth)  Example:  The Kennedys and the Vanderbuilts and the Rockefellers no longer have an amassed fortune.  It has been spent by their heirs and distributed morally (not by force) to society.