"Double dipping"
The problem is the mental illness that will precipitate in the airlines.
What about a QF pax who is an AAdvantage member flying from HKG on a QF code. oneworld rules are that privileges are available if the flight is both marketed and operated by oneworld carriers. If NZ stayed in Star, then the pax could get into CX's lounge if the flight was operated by QF but not if the flight was operated by NZ.
And pity the poor ground and cabin crew who are already struggling to keep abreast of one alliance's benefits without having to learn two completely different sets of rules.
That's before you start looking at planning problems. If an NZ flight could be scheduled to connect well with a UA AKL-LAX or a QF AKL-LAX, which should be given preference?
It was interesting to see the press releases denying that NZ is imminently leaving Star. Not a hint that QF might leave oneworld.