It all depends on marketing IMO.
With little or no marketing (which is what I expect)..I'll say 250k lifetime worldwide.
With good marketing (tv ads and instore) I'd say 1.5 million.
The fact they have predicted 500k..means they are planning to skimp on advertising...and means they are planning to have less units instore (which means the game will end up blending in on store shelves...meaning even fewer sales)....so I doubt they will even reach 500k.
You plan for low sales...and you'll get low sales.