Probably the most strategic way to use 50mm credit cards would be to use them destructively, rather than just for direct gain. (All of this is illegal as well

Probably the most strategic way to use 50mm credit cards would be to use them destructively, rather than just for direct gain. (All of this is illegal as well as immoral, but just presented so people can develop countermeasures) Know that using the credit cards will cause the accounts to get frozen, which will cause decreased purchasing; it will also scare people away from those stores, and possibly from purchasing in general. A nation state could do this for disruption directly; Russia could filter the 50mm cards to find cards belonging to US people (or just assume home depot = usa), and intentionally cause transactions requiring replacement. Do this on the last week before xmas, or black friday, or some other strategic time. A criminal organization could use the breach to manipulate the stock market -- either directly (shares in the breached company tank, although this doesn't happen to a very large extent), or by blocking cards used at one merchant in particular, raise the sales of a competitor indirectly. There's also straight extortion -- we'll sell these back to you and go away IFF you pay us.