That sounds completely reasonable - if you assume the users currently on Apollo 1) Can't be made to pay, directly with a price for the app or micropayments or indirectly via ads 2) Would move to the official app if Apollo shut down and 3) Don't bring in posts and comments with which other users engage, raising money that way.

Sounds to me that shutting it down is the one sure way to lose money, but I also don't trust that the reddit team had made that calculation properly. This all seems like a short term profit or like a power play to me.