An idea may solve a problem people care about, yet still be difficult to deliver or unable to cover its costs. These are separate tests. Desirability asks whether it is useful to the customer. Feasibility asks whether it can be delivered. Viability asks whether the economics can work.
Treating them separately makes uncertainty easier to investigate. Enthusiastic feedback does not establish delivery capability, and a working prototype does not establish demand. Evidence for one question should not quietly become an answer to all three.