For what it's worth I think that the new 9,000 space car park is very sensible planning by MAG. It simultaneously allows for the closure of the temporary car parks on Airport City and, if required, the closure and re-location of current parking on the T2 apron and on what will become (hopefully) the T3 apron. This means that the airport has felxibility in terms of growth of air movements which is what we all want.
Whilst I understand people pointing to Airport City as a location for long-term parking in the form of a mulit-story, I think the lead-in time is simply too long. Any car park here would be at the back of the development, allowing for higher end uses to be closer to the airport buildings. As such, it would be accessed via the new 'loop' road which is yet to start construction and would require the closure of the existing car parking on Airport City to even start. Going down this route could therefore put more pressure on available parking spaces when we want the apron clear of them.
In terms of how this fits in with the overall parking 'vision' for the airport, I know that MAG are looking are extending all of the Terminals directly linked parking via new multi-story car parks. At T2 this would be an obvious extension of the existing car park, at T3 an extension to the north and at T1 and new carpark in the area between the existing car park and The Station. This will presumably allow for the existing car park to be redeveloped either as a new car park or, more likely, as additional terminal facilities.
At Airport City i'm sure we will see multi-story's to support that development but I really do think this planned and approved 9,000 space car park is the right way to go.