There is other side to the story. If your app requires a backend server, it is financially prudent to pay a recurring fee that way hosting charges are taken care of and you can be somewhat sure the service will last as long as there are enough users.
There are plenty of games and apps that are dead because the developers no longer make new sales and the previous money is all spent and the business become unviable and they shutdown the servers.
Recurring charges gives the incentive for the developer to fix bugs and maintain the codebase and not abandon it completely once they loose interest, if original developer looses internest recurring revenue makes the code/service valuable so that a new developer can buy who makes enough in maintaining it.
If the app/service you bought does not have enough paying users to even pay for hosting servers or a part time developer time to maintain it, then no revenue model will save it anyway.