That would be some weird thinking from Boeing. You want to launch in 2022, well we can provide the aircraft you need in 2024 (maybe) but in the meantime have some short range smaller aircraft that has nothing to do with the order you are asking about. Let's hope that another airline doesn't use the competitor to launch the routes you wanted to to get a first mover advantage.
The only cities in Europe with sufficient yield and demand to support direct flights to and from Sydney/Melbourne are London, Paris and Frankfurt. I doubt that LH, AF or BA will buy a small subfleet of somewhat specialized aircraft just for those particular routes (where else could they go with such planes? Tahiti and Auckland lack the yields). BA would probably codeshare with Qantas. I doubt Virgin Atlantic have the resources to challenge Qantas on such routes either (and could they achieve enough yields?). Not to mention, all the European airlines have more profitable alternative opportunities that don't require specialized aircraft.
Admittedly this presumes it wouldn't be costless for a European airline to convert and A350-1000 into a ULR and then reconfigure the cabin into an appropriately high-yielding one for the route. This presumption seems fair to me though. Reconfiguring cabins is expensive, and airlines don't really like super-specialized subfleets.
It makes more sense, at least from what I've seen, for the European airlines to just codeshare on QF's metal. Or if they want to fly direct to Australia, Perth is at least theoretically feasible with current technology and standard LOPAs (at least for LH and maybe AF, but VX and BA would find things more difficult).