My personal take on this one is that AZ simply can't afford to leave, mainly considering its limited network (which has been shrinking significantly since the airline's decision to leave Milan MXP and base most of its aircraft at Rome FCO, leaning towards an overall unprofitable short/medium haul model) and moreover assuming that a good deal of revenue feeds from the JV itself. Even with an improved capacity in terms of long-haul operations (which I can't really see, at least for the time being), AZ would still benefit (in all respects) from being part of a wider group. Let's not even mention that EY's plan for AZ has so far been a disaster, with the Italian carrier losing scary figures of money and with rumours strongly insinuating that at least 15 aircraft (319/320) could soon be parked somewhere for good.
G