This is a great business model. There's no reason it has to be run by an airline for its own tickets. Anyone could set it up like an insurance company: tell us what the airline quoted and pay a fee. If the fare goes up in the next 24 hours, we buy it for you and you pay us the original fare (plus the fee you already paid). If it doesn't go up, or if you decide you don't want it, you're out the fee.

Someone would have to run the numbers, someone would have to figure out a way of confirming alleged fare quotes, and there might have to be exceptions for things like an advance-purchase fare quoted a few hours from the advance deadline, but it could be made to work on its own.