That's a very interesting question. I believe the answer is wrapped up in some issues of business culture.

They all have the money, they all have the vision, and they want the best. Clearly when it comes to buying hardware, they have no problem signing the cheques.

IMHO when it comes to business processes, then they are less aligned with (what looks to us like) best practice. You see symptoms of this exhibited in other areas - eg. EK's lunatic non-standard premium classes, QR's weird booking processes, bonkers FF programmes (EK an honourable exception) and ALL of their utterly garbage websites.

The very top management layer will always be occupied by a local, and a strategy they don't personally like is unlikely to get through. No doubt the airlines in question have made some evaluation of the alliances, in their own sweet way.

My money is on them not bothering. EK, EY and QR will rather use their sovereign wealth to buy market share and just front it out. But if one joins, they all will.