LOL... think about what you are saying. Where do you think the LAND comes from to build upon in built up areas? How do you think rezoning occurs? Do you think there are all these vacant blocks in the CBD waiting to be built upon?
Developers/investors often need to buy established property, sometimes many adjacent to each other, and often hold for a few years before major development. Hence, they need to be able to buy & hold existing property. Your theory would effectively put an end to that for many developments through foreign ownership prohibition.
You make a good point about Foreign investors wishing to buy several established adjoining properties to build new and I'm sure FIRB do ask the purpose behind the purchases of these established properties, again the point refers back to foreign investment into a newly constructed building and yes I see this as ok, as long as they build the thing.
Foreign investors buying for the purpose of land banking it away for many years with no intention of building anything or just putting together the DA just to on sell, I'm against.