It is economic because it will only take 4-8 of the top premium passengers to pay the fuel bill.
The aircraft can still carry cargo in the rear hold. The forward hold is deactivated as it reduces weight and fuel burn, it can be reactivated again.
They have a system in place also fast cabin changes so the seating configuration in the rear can be changed relatively quickly for seasonal demand.
Fuel bill will be on the order of $100,000 per fill up (i.e., one-way sector).
Yes, can carry some cargo in rear hold -- but you aren't going to tell a pax paying $10,000 for a ticket that they can't carry any (or many) checked bags.
Maybe you can shuffle the PE seats in the rear to regular economy -- but you ain't moving around the 80J seats very quickly. And is demand seasonal on NYC-SIN?