It will likely have very few takers apart from SIA. But it probably still make sense from Airbus's point of view. If you read the article that you linked to, you will see that from the end of this year all A350-900s will be built to a common production standard (the -900ULR spec) and it will be up to customers if they want to pay to activate the ULR capability (clearly most won't).

Except that there will probably be fewer A350-900ULRs than the 34 A340-500s built. :O

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?

Less fuel burn from the new engines, I think that they said 25% less burn

The big question is: If you really have enough pax demanding an ULR city pair, is it really more expensive to do it with an ULR aircraft model, compared to do it with a slightly cheaper conventional aircraft type, but do the stopover (requiring two cycles on the airframe, additional airport fees, two crews etc. potentially more time so that you need twice the number of aircraft to serve the route parking them for quite some hours)? Do you really have to charge a premium / 10K?

The number of routes is surely limited, but I can imagine that there are some where you could even offer the direct flight cheaper than your competitors doing a stopover. It all depends on the actual demand for that city pair, whether you need to fill the aircraft with transit passenger, or whether you can fill it with point-to-point passengers.
Having one model of an aircraft type for this relatively small market probably makes sense.

Exactly. The alternative to a 20 hour flight might be a 30 hour trip...
Which of course would be acceptable, if the 20 hour flight is 10K, and the 30 hour trip is 4K...

For sure the direct flight would be significantly less than twice as expensive to operate (maybe even cheaper), so you should not try to sell it at twice the price.
Maybe by doing so, you would just kill it.

I'll state the obvious : SST is way too expensive for it be economical and kill off the competition.
I'm not sure it would be economical as well.
You would need to save 40k$ (which is the cost of the flight hour of the cargo airplane) worth of fuel for it to be worth the trouble. Not taking into account the development of a civil air to air system from design to crew training.

A quick fuel and crew change stop, keeping the passengers onboard, would be a much more economical option. This would probably add 1 - 1.5 hours to the trip.

Done today by Ethiopian on several North American routes.

Also happened at Sal Island for many years during the embargo on South Africa, JNB to JFK in those days 18.5 hours, with a wife and two babies, not funny.

Understood in regard to current technology, but within a relatively short space of time, the capability should arrive unless the vested interests step heavily on any future development cycle....and yes, under every rock is a critter with your name on it.

IG

Even new technology cannot go against the laws of physics. Parasitic drag rises a lot after Mach 0.8 and decreases again slowly after Mach 1, but still stays higher than below Mach 0.8. The only solution to this is to go (sub) orbital, but then a lot of energy is needed to get there, which must be dissipated getting back. Plus you need to bring oxygen for the passengers to breathe and air breathing engines will not work.

I'm right Jack - but you pax stay in your seat and suffer.

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.

I don't think anyone is suggesting that conversion of an A350-900 between ULR and non-ULR configuration would be a turnround process. If done at all, it would be more likely something done on a seasonal basis. If at all.

But routes that are seasonal tend to be leisure-oriented. Business-heavy routes (the only ones that could possibly support ULR) tend not to be seasonal. Thus, I doubt that this A350-900 "flexibility" will be used -- except when converting ULR -900s back to regular once their previously flown ULR routes are discontinued due to impossible economics.

Or they might make so much money on SIN/EWR that they keep it in ULR configuration for ever. :O

That certainly wasn't the case when the tried it with the A340 (or the similar SIN/LAX service). I somewhat seriously looked at the LAX/SIN non-stop option one time - it was a massive premium over the similar business class service with a one and a half hour fuel stop thrown in (as in almost 2x).
Not to many people are willing to pay that much of a premium to save a couple hours, and many of those already have access to a private jet...

I've just been looking at average freight per flight on ULH routes (from US BTS T-100 data) for 2017 and not surprisingly it is not too high westbound and the forward hold will likely be empty:

UA LAX - SIN (789) 2.7 tonnes, average flight duration 17:07
AA DFW - HKG (77W) 2.3 tonnes, average flight duration 16:04

The results will obviously vary by season and indeed day depending upon the jetstream, route taken, holding, etc. However consider the eastbound route:

UA SIN - LAX 9.4 tonnes, avg flt dur 14:34
AA HKG - DFW 12.3 tonnes, avg flt dur 13:54

So you may well want to use the forward hold in one direction. Also what if the aircraft finds itself on a west coast to Tokyo rotation? You could dedicate an aircraft to ULH routes but that would reduce flexibility. I believe that Pan Am (and doubtless other operators) switched 727 QC between passenger & cargo configuration in a matter of hours but doubtless there was a weight penalty which negates one of the reasons d'etre for deactivating it.

My solution... It's already been discussed in another thread, but I believe that there is a lot of scope for utilising the forward hold with suites, toilets, galleys, etc releasing a lot of space for pax, although of course it will add weight.