Yes, the economic problem of reward absence is exclusive to open source and private software does not have it. They may have others, like excess of rewards to hackers in form of crypto ransom to the point that the defense department had to step in and ban payouts.
As long as business is not going as well as owners want, the same economic problem exists in private software too - in fact, the private companies get acquired all the time too, and they get shut down, causing DOS for many of their clients.
One difference is that closed-source source usually much less efficient; I cannot imagine "100K+" customers from a commercial org with just a single developer. And when there are dozens or hundreds of people involved, it's unlikely that new owners would turn to outright criminal activity like malware; they are much more likely to just shut down.
