It can still be a positive sum trade to purchase something from a corrupt and inefficient monopoly. Indeed generally the only reason corrupt and inefficient monopolies can stay in business is because having access to the good/service still has value to its customers, even if they'd be happier having an alternative.

Sure, VCs make a few negative sum trades when they invest in companies that aren't going to generate the exit they were hoping for, but they're aware of that and trying to balance them out with ridiculously positive sum investments anyway...