But that skews in the opposite direction: If they lost more users, that means they lose customers more quickly, so the value of a customer decreases the higher

But that skews in the opposite direction: If they lost more users, that means they lose customers more quickly, so the value of a customer decreases the higher you raise your estimate here. Losing more users is good in that it means they gained more in the same timespan - for fixed changes in the number of users (which is the datapoint we have). But it also means they are having a hard time keeping them.