its a matter of trust, as what users mentioned above. When users paid $50, they were promised that its a lifetime subscription. If they were told that it will be $50 for a limited time, I'm sure many users wouldn't have bought.
Just imagine you go to a car shop and you buy a car, then after 4 months after you paid, they tell you that the car you bought doesn't belong to you anymore, and you need to pay monthly to use it.