It's not rocket science. The airlines (and the manufacturers) know how many passengers currently fly between the ULR city pairs in question via hubs in Europe, ME3, Asia, etc.
By taking a view about the proportion of that traffic willing to pay a premium for direct, non-stop service on those routes, it's possible to derive the potential market for a ULR aircraft of a given size
The top ticket prices will be well above 10k.
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.
So when you compare you should probably compare the different trip options and not per se the long trip in isolation.
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.