It may sound banal, but I believe the limiting factor could be flight and duty time rules. The present rules, for Europe at least, are structured around what is flown currently, eg Tokyo, Singapore nonstops etc.
FTL rules are a battleground between airlines, pilot unions and regulators. The latest round of regulatory tweaking looks to be drawing to a close with heavy bruising on all sides. No way would the airlines want to reopen the debate for the sake of a questionable Europe-Australia capability.