If they barely had 10k in their vault, then it logically follows that they almost never need that much (otherwise they would obviously make it a point to keep m

If they barely had 10k in their vault, then it logically follows that they almost never need that much (otherwise they would obviously make it a point to keep more). If they rarely need that much, then it logically follows that somewhere at or less than 10k is a reasonable amount to have. There is no sense in taking the increased liability of having more if they don't need to. Furthermore, acting as a "proper bank" means they do a lot more than just acting as your personal piggy-bank/mattress. If they have all their money tied up in cash, it means they can't actually be preforming real bank activities (investment).