Not to mention that even if you could find addresses, if the block reward went to zero TODAY without a billion people using Bitcoin, even today's minute amounts of transaction fees would still be worth more per cpu/gpu/fpga cycle than if you'd spent that same cycle looking for populated addresses. Given that the same equipment you'd use in such an address search should, because of the similarity of the tasks, be capable of mining as well, it would be more profitable to use that equipment for mining purposes.