Even at 8 digits people could still use their birthdays. I think there's a tradeoff between PIN size and fraction of the population that will feel comfortable p

Even at 8 digits people could still use their birthdays. I think there's a tradeoff between PIN size and fraction of the population that will feel comfortable picking a number with no significance. I'd speculate that in a world where every PIN has to be 16 digits that almost everyone would just use 2 or 4 concatenated dates.