Chance are negligible. If collision occurs with a funded address, attacker you can transfer funds elsewhere.
Chances are still negligible when 1 billion people are using it? also can't I just run some kind of bots, that randomly generate addresses to see if
they have funds in them?
Yes, chances remain negligible. You could run your bot, but it'd be a waste of electricity. Chances are you'd wait the lifetime of the universe before finding a collision.
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.