While I'm sure most profits/margin come from first/business class, I think that's only viable because the large number of economic class passengers help marginalize the cost per passenger. To give a concrete example, suppose the airline is flying 20 business seats per flight. The cost of those seats to the airline if it had no economy class, and it were forced to allocate an entire plane for them would be immense -- marginal the cost per seat would be (illustration) ~Airplane_Cost/20+Accomodation_Cost. Now suppose a current airline will carry perhaps 200 economy class passengers where it makes 0 profit. The marginal cost of the business class seats to the airline now will be much lower, something like ~Airplane Cost/(200+20)+Accomodation_Cost.

So the airline don't need economy class to make profit, they need them to create economies of scale that bring down their cost for the high margin customers.

I have been on a 747 that was business class only (Singapore Airlines). But the economy class passengers are definitely required for scale on most flights.

If true, concord alone would be viable, but a combination of concord and regular planes on the same route would lead to all the business passengers taking concord, and the economy ones left in standard planes, which would no longer be viable.

In that case you could raise the price of Concorde tickets to push surplus business customers back to regular planes. But this raises the question, why would you even want to bother with Concorde in the first place?

It only makes sense if you could attract new business class customers, who would take the faster trip but wouldn't take the slower one at all.

Because you take them off the competition.

At the same time, when Concorde stopped flying after the accident in 2000, people who used to fly on her, flew on other things, so these passengers weren't lost revenue wise.

Some people didn't fly at all when they would've flown if a Concorde had been available. And those who did fly on other planes paid significantly less.

A significant share of those customers must have been lost by the two companies that flew the Concorde, as the alternatives to it are commoditized and offered by many other companies.

That was also the case back when Concord was in service...

Wouldn’t a more costly to produce product eliminate the large margin of existing first/business class?

And yet a number of "First/Business Class Only" airlines have failed.

But most of them failed because a jump in oil prices in 2006-2014. We're below these levels now.

For the moment. I don't know that I'd want to build a business - and certainly not an aircraft production line - on the assumption that oil will remain this cheap.

What airlines often do if they are worried about this is to hedge through oil futures.

Yeah but they lack hammocks!

There are planes with hammocks? Sign me up