The Westpac is already a dinosaur, like Eden Park. They were built for a crowd that existed twenty years ago, and was predicted to last a lot longer than it did.
It's done well but in terms of hosting top events we're a whole big cycle from another cricket, league or rugby world cup, the sports that we have attracted top level comps to NZ. FIFA won't look at us for anything more than a junior world cup and current capacity just about does for that.
Tied in with the potential for the ABs (the reason we've got these massive honking stadia that don't suit anyone who isn't them) to hit the skids, looking at a sensible stadium plan for NZ - smaller grounds with expandable capacity is the way forwards - oh, and specialising ffs. Rectangle grounds for rectangle fields, a decent cricket ground etc)