Has someone compared the cost of mining vs. that of brute-forcing addresses?
The chance of generating the key of a very rich address is effectively zero. That is, in the entire history of humanity, nobody has come anywhere close to accomplishing such a feat. 

Let's consider 1 Bitcoin a score. Since there can never be more than 21 million bitcoins or so, you need to hit on one of at most 21 million accounts. Let's say you could try 2 trillion accounts per second. Your odds of matching an account in a hundred years are approximately one in 11 billion billion.

Good luck.