yup. I'd be suspicious of any "unlimited" plan which didn't offer such a guarantee - anything that doesn't put its money where its mouth is is just marketing, a polite euphemism for "lies" in this case.
or time-bound it, so everyone's clear - if I know I'm buying a 10 year plan, that's fine. I know I'm getting value for money, and the service provider knows they have a finite projected cost.
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."
I'd think that such a warranty would be implied. It's a standard expectation that if you can't deliver what a customer paid for, you refund their money. If it's an ongoing service, then you can prorate the refund... but for a lifetime service, they've always used 0% of it and should thus get a 100% refund.
or time-bound it, so everyone's clear - if I know I'm buying a 10 year plan, that's fine. I know I'm getting value for money, and the service provider knows they have a finite projected cost.