Usually one tends to put it the other way, that economy pays for the trip and business class delivers the profit. Which makes sense because that is how e.g. a low cost carrier operates. While business class only flights are almost non-existent even though that in theory would make more money.
I think they're non-existant because there's not enough demand.
The travel group I work for caters for corporations and luxury travelers, and they'd stop selling economy in a heartbeat if they could (it's a money-loser for the company, as there's little to no commissions on them)
I don't think it has to do with demand as such. It is just hard to allocate resources with business class only flights. With a traditional layout you can oversell economy and upgrade or push people to the next flight. If you are running all your business class capacity on a few flights you can't do that. Nor can you have the same amount of routes. I guess you can say that is because of demand, but it is more because of the model. "Tourist business" doesn't really exist, unless maybe we count Norwegian.
That said, I would love to see an all business Airbus A321neoLR with Thompson Vantage Solo seats.
