So there going to be 2⁶⁴ possible addresses? Isn't it too few? Let's imagine your currency won the widest possible adoption and everybody (7*10⁹ users) on the planet uses it, and has private keys for 100 addresses (that's the minimum amount in bitcoin, actually we should consider much greater number). In that case, from each individual's point of view, collisions are still unlikely. But in general they are more than likely.

Another question - why coins are going to be indivisible? If that's just UI limitation, how divisible they are going to be by protocol?