Lots of devices attached to ongoing services today are sold for a one-time fee. For most of their existence, Microsoft and Adobe had very sustainable businesses based on one-time fees. It is a fine model. Software companies today are addicted to "recurring revenue" (thinking that is the only way to get sustainable cash flow), but a lot of people will pay you $20 once who would never pay you $1/month, and there will always be new people using popular products.
One-time fees are a perfectly fine model for a business, even if it is unfashionable currently.
One-time fees are a perfectly fine model for a business, even if it is unfashionable currently.