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.
It's the business travelers that are subsidizing the economy ones, not the other way round.
So it doesn't make sense to make economy more comfortable, actually the opposite.