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.
Just a little bit of experience in that matter:
For the EK 380 F-class luxury we heard the same arguments over and over. As a frequent visitor to the F-class i can assure you that less than 1/3 of customers pay the desired premium fare. Passengers smartly exploit over-bookings, upgrades through miles and more or ff programs. Such luxury has not payed and this can surely be extrapolated to a superduper ULR offer.
Fast cabin / cargo configuration changes have been offered in many other aircraft before. EK had additional tanks on their 77L, deactivated, offloaded, but never used as flexible fast changer. I remember Swissair tried seats that could be changed in width by FAs with a hand-drill, to enable C / Y configuration change between flights. It was quickly abolished.
The quick turn around times, the restricted time for maintenance simply does not favour such configuration changes, too much time to lose and too many blunders while changing (never touch working mechanics!). Why is the 77W, and most probably the 359 such a success? Because they are so versatile without touching anything. Sure enough a 380 carries more into slot restricted LHR, sure enough that torture version of the 350 might fly from Mongolia to Fireland, but its all niche. Niche was never economical, only viable if there was really no alternative. There is however an alternative to 18 hour travel: Split it. Its cheaper and after first-hand experience more comfortable.
The very few hard pressed business gurus might use superduper ULR, but there are not that many and they rarely pay premium.
I predict a few spectacular routes for such aircraft, it has been tried before and they all silently disappeared after some time.