The other way to look at it is that the service provider is borrowing money from you and paying you in services instead of interest. They have the right to pay the principal of the loan back at any time.
If their cost to provide the service is below the market cost of loaning them money, the difference is their margin. They simply need to price the "lifetime service" appropriately such that there's a market demand for it. If the price the market will pay is too little relative to their cost to provide the service, then they don't have a viable "lifetime offering."
If their cost to provide the service is below the market cost of loaning them money, the difference is their margin. They simply need to price the "lifetime service" appropriately such that there's a market demand for it. If the price the market will pay is too little relative to their cost to provide the service, then they don't have a viable "lifetime offering."